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Green tea (Camellia sinensis)



Interactions

Green tea/Drug Interactions:
  • NoteNote: Several of the following drug interactions associated with green tea are based upon the adverse effect profile of caffeine.
  • AdenosineAdenosine: Caffeine acts via blockade of adenosine receptors, and theoretically, may antagonize the effects of adenosine (111).
  • AlcoholAlcohol: According to a review, alcohol consumption may increase caffeine serum concentrations and the risk of caffeine adverse effects (70).
  • Alzheimer's agentsAlzheimer's agents: Several preliminary studies have examined the effects of caffeine, tea, or coffee use on short and long-term memory and alertness (268; 269). Although it remains unclear if green tea is beneficial for this use, there is a potential for additive effects between green tea and Alzheimer's agents.
  • AnalgesicsAnalgesics: In human study, caffeine enhanced the analgesic efficacy of acetaminophen (by 40%) in the relief of pain of various etiologies, such as uterine cramping, episiotomies, and dental extractions (71; 72). In human study, caffeine plus codeine also enhanced acetaminophen in reducing dental and oral surgery pain (73). In the treatment of menstruation-associated migraine and migraines, a combination of acetaminophen, aspirin, and caffeine was found to be highly effective in treating pain and disability (74; 75). Experts warn that there is an increased risk of toxic effects including liver damage with combined use. There are, however, conflicting results from human studies indicating a lack of additive effect and risk of hepatotoxicity (76).
  • AntiandrogensAntiandrogens: According to preliminary study, caffeine may inhibit drugs metabolized by CYP1A2 like flutamide, an antiandrogen (270). Theoretically, green tea may interact with antiandrogens.
  • AntiarthriticsAntiarthritics: In vitro, epigallocatechin gallate (EGCG), a component of green tea, inhibited IL-1 beta-induced expression of matrix metalloproteinase-1 and -13 in human chondrocytes, as well as activated the mitogen activated protein kinase subgroup c-Jun N-terminal kinase (271; 272). EGCG also inhibits the IL-1 beta-induced activity and expression of cyclooxygenase-2 and nitric oxide synthase-2 in human chondrocytes, as well as the production of nitric oxide (273; 274). Other in vitro (275) and animal studies (276) have also indicated anti inflammatory properties of green tea.
  • AntiasthmaticsAntiasthmatics: Research has shown caffeine to cause improvements in airflow to the lungs (bronchodilation). However, it is not clear if caffeine or tea consumption has significant benefits in people with asthma. In human study, caffeine consumption, such as coffee, did not affect exhaled nitric oxide in asthmatics (277). Other clinical research suggests caffeine may reduce theophylline clearance, increase elimination half-life, and increase serum levels (278).
  • AntibioticsAntibiotics: In human study, ciprofloxacin inhibited caffeine elimination (279; 280), and slightly increased the half-lifeof caffeine from 5.2 hours to 8.2 hours (281; 280). In human study, enoxacin (no longer available in the United States) significantly inhibited caffeine elimination (279). Caffeine clearance was decreased in patients treated with norfloxacin (282). Quinolones may inhibit the N-demethylation pathway of caffeine; however, in human study, afloxacin had no effect on the clearance of caffeine (279; 283).
  • Anticoagulants and antiplateletsAnticoagulants and antiplatelets: In in vitro and animal study, both catechins and caffeine in green tea exhibited antiplatelet activity (62; 63). In one case report, large amounts (one-half to one gallon) of green tea antagonized the effects of warfarin, possibly due to the small amounts of vitamin K in green tea (64; 65). In laboratory study, dry green tea leaves contained significantly more vitamin K than black tea leaves; green tea may contain 1,428mcg vitamin K per 100g leaf, while black tea may contain only 262mcg vitamin K per 100g leaf (66). In human study, caffeine inhibited the hemodynamic response to dipyridamole infusion in a dose-dependent fashion (257).
  • AnticonvulsantsAnticonvulsants: In humans, phenytoin induces CYP3A4 and has been shown to increase the clearance of caffeine (284).
  • Antidepressants, monoamine oxidase inhibitors (MAOIs)Antidepressants, monoamine oxidase inhibitors (MAOIs): According to preliminary study, caffeine and caffeine analogues are inhibitors of monoamine oxidase and concomitant administration with monoamine oxidase inhibitors (MAOI) may increase the risk of hypertensive crisis (285).
  • AntidiabeticsAntidiabetics: In human study, caffeine modulated blood sugar (52); however, this was not shown in various clinical trials (150; 149; 159). Green tea extract decreased streptozotocin-induced increases in blood glucose in rats (201).
  • AntiestrogensAntiestrogens: Based on epidemiological evidence, a relationship between green tea consumption and decreased estrogen levels in humans may exist (60). Based on human evidence, in combination with chasteberry, as well as with various vitamins, minerals, and amino acids, green tea has been associated with increased success in helping women to conceive (286). In human study, green tea in combination with other agents (such as black cohosh, soy, and kava) has been associated with relieving menopausal symptoms, including hot flashes and sleep disturbances (287).
  • AntifungalsAntifungals: In human study, fluconazole decreased caffeine clearance from the blood (77). In human study, terbinafine decreased the clearance of intravenous caffeine (258).
  • Antiglaucoma agentsAntiglaucoma agents: In human study, ingestion of caffeine increased intraocular pressure in patients with glaucoma (78). However, other clinical reports have not shown any significant effects of caffeine on intraocular pressure (79; 80).
  • AntihypertensivesAntihypertensives: In clinical study, green tea (81), green tea extracts (82), and specifically caffeine (83; 84; 85; 86; 87; 88; 89; 90; 91), increased systolic and diastolic blood pressure. However, there is controversy in this area and several clinical trials have not shown these effects (92; 93; 94; 95), or have demonstrated blood pressure-lowering effects (96).
  • Anti inflammatoriesAnti inflammatories: In vitro, EGCG, a component of green tea, inhibited IL-1 beta-induced expression of matrix metalloproteinase-1 and -13 in human chondrocytes, as well as activated the mitogen activated protein kinase subgroup c-Jun N-terminal kinase (271; 272). EGCG also inhibits the IL-1 beta-induced activity and expression of cyclooxygenase-2 and nitric oxide synthase-2 in human chondrocytes, as well as the production of nitric oxide (273; 274). Other in vitro (275) and animal studies (276) have also indicated anti inflammatory properties of green tea. A greater reduction of pain (2.4-2.8 times) occurred when ibuprofen was used in combination with caffeine, in comparison with ibuprofen alone (288). The combination of ibuprofen and caffeine had a greater analgesic effect for tension-type headache in a clinical trial (289).
  • AntilipemicsAntilipemics: In in vitro (17) and human (290; 291; 156; 153; 155; 292) studies, green tea and green tea catechins exhibited antilipemic effects. A lack of effect on lipid profiles, however, has also been reported in human trials using green tea (293; 294), or caffeine alone (295; 296; 297; 298). In humans, theaflavin-enriched green tea decreased levels of total and LDL cholesterol (153).
  • AntineoplasticsAntineoplastics: Various reviews (299; 300; 301), human studies (302; 303; 142; 35; 144; 36; 140; 141; 82; 304; 305; 306; 307; 138; 308; 309; 310; 311; 135; 312), and in vitro and in vivo studies (313; 314; 315; 316; 317; 318; 319; 320; 321; 322; 25; 323; 324; 325; 326; 327; 328; 171; 329; 330; 331; 332; 333; 334; 335; 336; 337; 338; 339; 340; 341; 342; 343; 344; 345), have indicated mixed results in the reduction of cancer risk with use of green tea. The combination of green tea and doxorubicin induced a 37% reduction in tumor weight and a 2.5 fold increase in doxorubicin's inhibitory effect on tumor growth in mice (346). There was no increase in doxorubicin concentration in normal tissue. In vitro, EGCG modulated apoptosis induced by gemcitabine (GEM), mitomycin C, or 5-fluorouracil and mitochondrial membrane depolarization, cytosolic cytochrome c expression, and apoptosis were increased in cells incubated with EGCG and GEM compared with either agent alone in KMCH, CC-LP-1, and Mz-ChA-1 human cholangiocarcinoma cells (347). In one study, it was determined that among patients with cancer, green tea was included as an herbal remedy most often used that had potential interactions with the chemotherapy they were also receiving due to similar metabolism via the cytochrome P-450 metabolizing enzymes (CYPs) and/or the P-glycoprotein (P-gp) transporter (31).
  • Antiobesity agentsAntiobesity agents: In human study, green tea extract (157; 96) and supplements (348) exerted antiobesity effects. In human study, green tea in combination with other herbal agents, such as bitter orange and guarana, increased carbohydrate oxidation (349).
  • AntipsychoticsAntipsychotics: Coffee and tea withdrawal does not appear to increase the bioavailability of chlorpromazine, haloperidol, fluphenazine, and trifluoperazine or affect the individual variation in plasma levels (350).
  • AspirinAspirin: Caffeine does not enhance the analgesic effect of aspirin according to a meta-analysis (351). However, in a clinical trial studying treatment of menstruation-associated migraine, a combination of acetaminophen, aspirin, and caffeine was found to be highly effective in treating pain and disability (74). Others report that the addition of caffeine to aspirin has significant benefits on mood and performance (352). A combination of aspirin, butalbital, caffeine, and codeine has been found superior to acetaminophen plus codeine in relieving oral surgery pain (73).
  • Antiulcer agentsAntiulcer agents: In human study, when taken with caffeine, cimetidine (Tagamet®) increased caffeine blood levels or the length of time caffeine acts on the body (97).
  • Antiviral agentsAntiviral agents: In human study, daily intake of capsulated green tea diminished human T-cell lymphocytic virus (HTLV-1) provirus load (152). In human study, treatment with Polyphenon E® (a proprietary extract of green tea leaves) resulted in clearance of genital warts (163; 164; 166). In vitro, EGCG inhibited HIV-1 infectivity of human CD4(+) T cells and macrophages and at a physiologic concentration of 6mcmol/L, EGCG significantly inhibited HIV-1 p24 antigen production across a broad spectrum of both HIV-1 clinical isolates and laboratory-adapted subtypes (353). EGCG has also been shown to degrade semen-derived enhancer of virus infection (SEVI) and inhibits SEV1 activity leading to decreased semen-mediated enhancement of HIV-1 infection (354).
  • BarbituratesBarbiturates: In animal study, caffeine has been shown to shorten barbital-induced sleeping time (355).
  • BenzodiazepinesBenzodiazepines: In human study, caffeine (125-500mg) counteracted both the effect of reducing anxiety and the reduction in mental performance associated with 2.5mg lorazepam (98). In clinical study, caffeine antagonized the effects of diazepam (99; 100). In animal study, one week of treatment with green tea extract may increase C(max) and AUC(0-infinity) of orally administered midazolam without changing the half-life (356).
  • Beta-agonistsBeta-agonists: Theoretically, caffeine may add to effects of stimulants, such as beta-adrenergic agonists.
  • CaffeineCaffeine: Green tea is a source of caffeine. Theoretically, there may be additive side effects with other products containing caffeine.
  • Calcium saltsCalcium salts: In clinical study, caffeine (400mg daily) may not affect levels of calcium absorption, endogenous fecal calcium, or urinary calcium in premenopausal females (67); however, evidence from a review suggests that caffeine may increase urinary calcium levels (357).
  • Cardiovascular agentsCardiovascular agents: According to various reviews (358; 12; 359; 292), and animal and human studies, green tea and/or its constituents, may increase blood pressure, reduce LDL oxidation, inhibit platelet activation and aggregation, decrease cholesterol levels, and modify endothelial activity (360; 361; 362; 363; 364; 365; 366; 367). Verapamil increases plasma caffeine concentrations (368). Based on human evidence, mexiletine may decrease caffeine elimination by 50% (369). Caffeine acts via blockade of adenosine receptors, and theoretically, may antagonize the effects of adenosine (111).
  • CimetidineCimetidine: When taken with caffeine, cimetidine (Tagamet®) may increase caffeine blood levels or the length of time caffeine acts on the body (97).
  • ClozapineClozapine: High doses of caffeine (400-1,000mg daily) may inhibit clozapine metabolism (via CYP1A2) to an extent, which may yield clinically significant elevations in clozapine levels for some individuals (370). In animal study, administration of green tea extract did not alter the elimination half-life of clozapine; however, the time to reach peak concentration was increased and the maximal peak plasma concentration of clozapine was lower following green tea extract administration (371).
  • CNS StimulantsCNS Stimulants: Theoretically, caffeine may add to effects of stimulants, such as beta adrenergic agonists. A case report cited ischemic stroke after the nasal ingestion of amphetamine and caffeine (101).
  • ContraceptivesContraceptives: In clinical study, estrogen inhibited CYP1A2 mediated caffeine metabolism (102). Estrogen contained in oral contraceptives (OCP) has been shown to lengthen the half-life of caffeine. In human study, an extended half-life was demonstratedin healthy female volunteers who used oral contraceptives for greater than three months (7.88 hours), compared to a control group (5.37 hours) (103).
  • CorticosteroidsCorticosteroids: Theoretically, caffeine levels may be lowered by taking dexamethasone (Decadron®). In clinical study, caffeine enhanced topically applied hydrocortisone in the treatment of atopic dermatitis (372).
  • Cytochrome P450metabolized agentsCytochrome P450metabolized agents: Based on animal and laboratory study, cytochrome P450 1A2 is involved in the metabolism of caffeine (373; 374; 375). According to reviews, tea has been reported to induce cytochrome P450 enzymes 1A1, 1A2, 2B1, and 3A4 (376; 377; 378). However, in one study in healthy humans, green tea extract did not alter the pharmacokinetics of two cytochrome P450 3A4 or 2D6 probe drugs, dextromethorphan and alprazolam (379). Another study also showed that green tea extract did not affect the activity of CYP1A2, CYP2D6, CYP2C9, and CYP3A4 (378). In clinical study, high doses of caffeine (400-1,000mg daily) inhibited clozapine metabolism (via CYP1A2) to an extent, which may yield clinically significant elevations in clozapine levels for some individuals (370). In human study, disulfiram consumption for at least four days was associated with a significantly increased half-life (4.1-5.7 hours, p<0.01) and decreased clearance (142-99mL per min, p<0.005) of caffeine in normal volunteers (380). Fluvoxamine is a potent inhibitor of caffeine metabolism via the cytochrome P450 system. Therefore, patients being treated with fluvoxamine may be at an increased risk of caffeine adverse effects, according to a clinical trial (381). According to secondary sources, caffeine and riluzole are both metabolized by cytochrome P450 1A2. In in vitro study, green tea extract, but not epigallocatechin gallate, induced CYP1A2 mRNA expression, whereas CYP1A1 and CYP3A4 mRNA expression may not be affected (382).
  • DecongestantsDecongestants: Blood pressure elevating effects may be synergistic when caffeine is combined with phenylpropanolamine (PPA) (no longer available in the United States). According to a review and a survey, PPA increased plasma caffeine levels approximately four fold (106; 107). The combination of PPA and caffeine induced psychotic symptoms in one woman with no previous history of mental illness (106).
  • Dental and periodontal agentsDental and periodontal agents: ECGC has been shown to reduce the progression of dental erosion in a clinical trial, possibly mediated through its ability to inhibit MMP enzymes (383). In another clinical trial, green tea was examined as a rinsing aid to prevent bacteria in the oral cavity (384).
  • DipyridamoleDipyridamole: Caffeine appears to inhibit the hemodynamic response to dipyridamole infusion in a dose-dependent fashion (257).
  • DisulfiramDisulfiram: In human study, disulfiram consumption may be associated with an increased half-life (4.1-5.7 hours, p<0.01) and decreased clearance (142-99mL per min, p<0.005) of caffeine in normal volunteers (380).
  • DiureticsDiuretics: Green tea containing caffeine may produce a diuretic effect. Clinical research suggests that a reduction of caffeine to less than 100mg daily significantly reduced the episodes of leakage daily in patients with symptoms of urinary frequency or urge incontinence (108). Dysuria has been reported in secondary sources.
  • DoxorubicinDoxorubicin: In animal study, the combination of green tea and doxorubicin induced a reduction in tumor weight and an increase in doxorubicin's inhibitory effect on tumor growth in Ehrlich ascites carcinoma tumor bearing mice (346). There was no increase in doxorubicin concentration in normal tissue. Theanine, a component of green tea, has been suggested to reduce the adverse reactions of doxorubicin (385).
  • Drugs that may lower seizure threshold Drugs that may lower seizure threshold: In a case series of six depressed patients receiving electroconvulsive therapy (ECT), the effect of caffeine increased the length of the seizure and potentially enhanced the therapeutic effect (109). Also, in a review of case reports, seizures have been reported from caffeine overdose (110).
  • EnoxacinEnoxacin: Enoxacin (no longer available in the United States) may inhibit caffeine elimination (279).
  • EphedrineEphedrine: Caffeine may enhance the thermogenic activity of ephedrine (112; 113; 114). Other adverse effects due to the combination include abnormal heart rhythms, insomnia, anxiety, headache, irritability, poor concentration, blurred vision, and dizziness.
  • Ergot derivativesErgot derivatives: According to a report, caffeine may enhance the effectiveness of ergot medications in the treatment of migraine headaches (Cafergot®: ergotamine tartrate, USP 1mg, and caffeine, USP 10mg) (386).
  • EstrogensEstrogens: Estrogen has been shown to inhibit CYP1A2 mediated caffeine metabolism (102). Estrogen contained in oral contraceptives (OCP) and hormone replacement therapy has been shown to lengthen the half-life of caffeine. One study has demonstrated an extended t˝ in healthy female volunteers who used oral contraceptives for greater than three months (7.88 hours), compared to a control group (5.37 hours) (103).
  • Fertility agentsFertility agents: Early research using a combination product containing green tea, called FertilityBlend, has been associated with some success in helping women to conceive (286). However, there is a lack of available study investigating the use of green tea alone for fertility.
  • FluvoxamineFluvoxamine: Fluvoxamine is a potent inhibitor of caffeine metabolism via the cytochrome P450 system. Therefore, patients being treated with fluvoxamine may be at an increased risk of caffeine adverse effects (381).
  • Hematological agentsHematological agents: In clinical study, impaired iron metabolism and microcytic anemia occurred in infants (57). In one case study, green tea containing 250mg ascorbic acid per 100g tea induced transient acceleration of intravascular hemolysis in a 67 year-old man with paroxysmal nocturnal hemoglobinuria (58).
  • Hepatotoxic agentsHepatotoxic agents: According to various case reports and a review, hepatotoxicity (38; 39; 40; 41; 23) and acute hepatitis (42; 43) has been associated with use of green tea products.
  • Hormonal agentsHormonal agents: Epidemiological study suggests a relationship between green tea consumption and decreased estrogen levels in humans (60). Based on clinical evidence, in combination with chasteberry, as well as with various vitamins, minerals, and amino acids, green tea may be associated with increased success in helping women to conceive (286). In human study, green tea in combination with other agents (such as black cohosh, soy, and kava) has been associated with relieving menopausal symptoms, including hot flashes and sleep disturbances (287).
  • IbuprofenIbuprofen: Based on clinical evidence, a greater reduction of pain may occur when ibuprofen is used in combination with caffeine, in comparison with ibuprofen alone (288). The combination of ibuprofen and caffeine may have a greater analgesic effect for tension-type headache (289).
  • ImmunosuppressantsImmunosuppressants: Based on clinical evidence, an increase in proliferation of gamma-delta T cells from subjects taking green tea, and secretion of IFN-gamma may occur (147).
  • Iron saltsIron salts: In clinical study, in thalassemia major and intermedia patients, tea caused an inhibition of iron absorption (55). However, it has also been suggested that green tea does not inhibit iron absorption (387). Impaired iron metabolism and microcytic anemia may occur in infants of breastfeeding women consuming caffeine (56). According to a review, it is recommended that vegetarians drink tea between meals, as tea binds to dietary iron found in plant sources and decreases the availability (absorption) of iron (59).
  • LevodopaLevodopa: In human study, pretreatment with caffeine improved the pharmacokinetics of levodopa in patients with Parkinson's disease (388). However, other study has found that caffeine did not potentiate the actions of these drugs (389).
  • LithiumLithium: In human study, caffeine withdrawal produced lithium toxicity in patients maintained at high lithium blood levels (252). In such patients, abrupt discontinuation of daily caffeine has resulted in a significant increase (24%) in lithium blood levels.
  • MethoxsalenMethoxsalen: Based on human evidence, methoxsalen may decrease the clearance of caffeine, resulting in a marked increase in t˝, from 5.6 hours to 57 hours (390).
  • MexiletineMexiletine: Mexiletine may decrease caffeine elimination by 50% (369).
  • Neurologic agentsNeurologic agents: In clinical study, use of green tea or constituents of green tea such as caffeine, may result in dizziness, headache, and nervous system stimulation resulting in symptoms such as delirium, confusion, restlessness, and insomnia (211; 141; 133; 175). Caffeine withdrawal may produce lithium toxicity in patients maintained at high lithium blood levels (252). In such patients, abrupt discontinuation of daily caffeine has resulted in a significant increase (24%) in lithium blood levels. Based on in vitro and animal evidence, EGCG may exert neuroprotective effects in models of Alzheimer's and Parkinson diseases (391; 392; 393).
  • NicotineNicotine: Additive effects on cardiovascular parameters may occur with nicotine (91). Concomitant consumption of caffeine and cigarettes during pregnancy may place the developing fetus at higher risk for diminished growth (394).
  • Nonsteroidal anti inflammatories (NSAIDs), COX 2 inhibitorsNonsteroidal anti inflammatories (NSAIDs), COX 2 inhibitors: In laboratory study, caffeine inhibited cyclooxygenase-2 (COX-2) and may be useful as an adjuvant analgesic agent (395).
  • P-glycoprotein regulated drugsP-glycoprotein regulated drugs: In in vitro study, green tea polyphenols may inhibit p-glycoprotein activity (396; 397). The involvement of green tea in p-glycoprotein regulation has been reviewed (398; 33). In human study, it was determined that among patients with cancer, green tea was included as an herbal remedy most often used that had potential interactions with the chemotherapy they were also receiving due to similar metabolism via the cytochrome P-450 metabolizing enzymes (CYPs) and/or the P-glycoprotein (P-gp) transporter (31).
  • Phenylpropanolamine (PPA)Phenylpropanolamine (PPA): Blood pressure-elevating effects may be synergistic when caffeine is combined with PPA (no longer available in the United States). PPA has been shown to increase plasma caffeine levels approximately four fold (106; 107). The combination of PPA and caffeine induced psychotic symptoms in one woman with no previous history of mental illness (106).
  • Proton pump inhibitors (PPIs)Proton pump inhibitors (PPIs): When administered in therapeutically recommended doses, PPIs do not appear to induce CYP1A2 or alter caffeine metabolism (399).
  • QuinolonesQuinolones: In human study, caffeine clearance decreased in patients treated with norfloxacin (282). Quinolones may inhibit the N-demethylation pathway of caffeine. Ciprofloxacin slightly increases the t˝ of caffeine from 5.2 hours to 8.2 hours (281; 280). Ofloxacin had no effect on the clearance of caffeine (279; 283).
  • RiluzoleRiluzole: Caffeine and riluzole are both metabolized by cytochrome P450 1A2 (400).
  • Sulfotransferase 1A3 (SULT1A3) substratesSulfotransferase 1A3 (SULT1A3) substrates: In in vitro study, green tea inhibited the sulfation of dopamine and ritodine, two substrates of SULT1A3 (401).
  • SympathomimeticsSympathomimetics: In clinical study, caffeine enhanced the thermogenic activity of ephedrine (112; 113; 114). Other adverse effects due to the combination include abnormal heart rhythms, insomnia, anxiety, headache, irritability, poor concentration, blurred vision, and dizziness.
  • TamoxifenTamoxifen: In vitro, a combination of green tea and tamoxifen inhibited the proliferation of estrogen receptor-positive breast cancer cells over tamoxifen alone (402).
  • TerbinafineTerbinafine: In humans, terbinafine decreased the clearance of intravenous caffeine (258).
  • TheophyllineTheophylline: In humans, caffeine reduced theophylline clearance, increased elimination half-life, and increased serum levels (278).
  • Timolol maleateTimolol maleate: Caffeine consumption does not appear to affect timolol treatment of the eyes (403).
  • VasodilatorsVasodilators: In human study, both caffeine and theophylline attenuated adenosine-induced vasodilation (111).
  • VasopressorsVasopressors: In human study, both caffeine and theophylline attenuated adenosine-induced vasodilation (111).

Green tea/Herb/Supplement Interactions:
  • NoteNote: Several of the following herb and supplement interactions associated with green tea are predominantly based upon the adverse effect profile of caffeine.
  • AlkaloidsAlkaloids: According to a report, caffeine may enhance the effectiveness of ergot derivatives (386).
  • Alzheimer's agentsAlzheimer's agents: Several preliminary studies have examined the effects of caffeine, tea, or coffee use on short and long-term memory and alertness (268; 269). Although it remains unclear if green tea is beneficial for this use, there is a potential for additive effects of green tea and Alzheimer's agents.
  • AnalgesicsAnalgesics: In clinical study, caffeine enhanced the efficacy of various analgesics in the relief of pain of various etiologies, such as uterine cramping, episiotomies, and dental extractions (71; 72). In human study, caffeine plus codeine enhanced an analgesic in reducing dental and oral surgery pain (73). In the treatment of menstruation-associated migraine and migraines, a combination of an analgesic, an anti inflammatory, and caffeine was found to be highly effective in treating pain and disability (74; 75). Experts warn that there is an increased risk of toxic effects including liver damage with combined use. There are, however, conflicting results from human studies indicating a lack of additive effect and risk of hepatotoxicity (76).
  • AntiandrogensAntiandrogens: According to preliminary study, caffeine may inhibit drugs metabolized by CYP1A2 like flutamide, an antiandrogen (270). Theoretically, green tea may interact with antiandrogens.
  • AntiarthriticsAntiarthritics: In vitro, epigallocatechin gallate (EGCG), a component of green tea, inhibited IL-1 beta-induced expression of matrix metalloproteinase-1 and -13 in human chondrocytes, as well as activated the mitogen activated protein kinase subgroup c-Jun N-terminal kinase (271; 272). EGCG also inhibits the IL-1 beta-induced activity and expression of cyclooxygenase-2 and nitric oxide synthase-2 in human chondrocytes, as well as the production of nitric oxide (273; 274). Other in vitro (275) and animal studies (276) have also indicated anti inflammatory properties of green tea.
  • AntiasthmaticsAntiasthmatics: Research has shown caffeine to cause improvements in airflow to the lungs (bronchodilation). However, it is not clear if caffeine or tea consumption has significant benefits in people with asthma. In human study, caffeine consumption, such as coffee, did not affect exhaled nitric oxide in asthmatics (277). Other clinical research suggests that caffeine may reduce the clearance of bronchodilators, increase elimination half-life, and increase serum levels (278).
  • AntibacterialsAntibacterials: In human study, antibacterials inhibited caffeine elimination (279; 280; 282; 281; 283).
  • Anticoagulants and antiplateletsAnticoagulants and antiplatelets: In in vitro and animal study, both catechins and caffeine in green tea exhibited antiplatelet activity (62; 63). In one case report, large amounts (one-half to one gallon) of green tea antagonized the effects of an anticoagulant, possibly due to the small amounts of vitamin K in green tea (64; 65). In laboratory study, dry green tea leaves contained significantly more vitamin K than black tea leaves; green tea may contain 1,428mcg vitamin K per 100g leaf, while black tea may contain only 262mcg vitamin K per 100g leaf (66). In human study, caffeine inhibited the hemodynamic response to an antiplatelet infusion in a dose-dependent fashion (257).
  • AnticonvulsantsAnticonvulsants: In humans, phenytoin induces CYP3A4 and has been shown to increase the clearance of caffeine (284).
  • Antidepressant agents, monoamine oxidase inhibitors (MAOIs)Antidepressant agents, monoamine oxidase inhibitors (MAOIs): According to preliminary study, caffeine and caffeine analogues are inhibitors of monoamine oxidase and concomitant administration with monoamine oxidase inhibitors (MAOI) may increase the risk of hypertensive crisis (285).
  • AntiestrogensAntiestrogens: Epidemiological study suggests a relationship between green tea consumption and decreased estrogen levels in humans (60). In combination with chasteberry, as well as various vitamins, minerals, and amino acids, green tea has been associated with increased success in helping women to conceive (286). In human study, in combination with agents (such as black cohosh, soy, and kava), green tea has been associated with relieving menopausal symptoms, including hot flashes and sleep disturbances (287).
  • AntifungalsAntifungals: In human study, antifungals have been shown to decrease caffeine clearance from the blood (77).
  • Anti inflammatoriesAnti inflammatories: In vitro, EGCG, a component of green tea, inhibited IL-1 beta-induced expression of matrix metalloproteinase-1 and -13 in human chondrocytes, as well as activated the mitogen activated protein kinase subgroup c-Jun N-terminal kinase (271; 272). EGCG also inhibits the IL-1 beta-induced activity and expression of cyclooxygenase-2 and nitric oxide synthase-2 in human chondrocytes, as well as the production of nitric oxide (273; 274). Other in vitro (275) and animal studies (276) have also indicated anti inflammatory properties of green tea.
  • AntilipemicsAntilipemics: In in vitro (17) and human (290; 291; 156; 153; 155; 292) studies, green tea or green tea catechins exhibited antilipemic effects. A lack of effect on lipid profiles, however, has also been reported in human trials using green tea (293; 294), or caffeine alone (295; 296; 297; 298). In human study, theaflavin-enriched green tea decreased levels of total and LDL cholesterol (153). One systematic review found that green tea reduced LDL levels (-0.23mmol/L; -0.34, -0.12; 4 studies) (290).
  • AntineoplasticsAntineoplastics: Various reviews (299; 300; 301), human studies (302; 303; 142; 35; 144; 36; 140; 141; 82; 304; 305; 306; 307; 138; 308; 309; 310; 311; 135; 312), and in vitro and in vivo studies (313; 314; 315; 316; 317; 318; 319; 320; 321; 322; 25; 323; 324; 325; 326; 327; 328; 171; 329; 330; 331; 332; 333; 334; 335; 336; 337; 338; 339; 340; 341; 342; 343; 344; 345) have indicated mixed results in the reduction of cancer risk with use of green tea. In one study, it was determined that among patients with cancer, green tea was included as an herbal remedy most often used that had potential interactions with the chemotherapy they were also receiving due to similar metabolism via the cytochrome P-450 metabolizing enzymes (CYPs) and/or the P-glycoprotein (P-gp) transporter (31).
  • Antiobesity herbs and supplementsAntiobesity herbs and supplements: In clinical study, green tea extract (157; 96) and supplements (348) exhibited antiobesity effects. In human study, green tea in combination with other herbal agents, such as bitter orange and guarana, increased carbohydrate oxidation (349).
  • AntioxidantsAntioxidants: Certain components of green tea, such as polyphenols, act as antioxidants. In clinical study, green tea, alone or in combination, increased the total antioxidant potential in hypercholesterolemic patients (154).
  • Antiulcer herbs and supplementsAntiulcer herbs and supplements: In human study, when taken with caffeine, antiulcer agents increased caffeine blood levels or the length of time caffeine acts on the body (97).
  • AntiviralsAntivirals: In human study, daily intake of capsulated green tea diminished HTLV-1 provirus load as compared with the controls in a subgroup of patients (higher provirus load) (p=0.031) (152). In clinical study, treatment with Polyphenon E® resulted in clearance of genital warts (163; 164; 166). In vitro, EGCG inhibited HIV-1 infectivity of human CD4(+) T cells and macrophages and at a physiologic concentration of 6mcmol/L, EGCG significantly inhibited HIV-1 p24 antigen production across a broad spectrum of both HIV-1 clinical isolates and laboratory-adapted subtypes (353). EGCG has also been shown to degrade semen-derived enhancer of virus infection (SEVI) and inhibits SEV1 activity leading to decreased semen-mediated enhancement of HIV-1 infection (354).
  • Ascorbic acidAscorbic acid: In clinical study, single doses of tea catechins decreased ascorbate levels briefly (404). In other clinical study, however, use of a green tea extract did not affect ascorbic acid levels (294). In a crossover study, supplementation with a nutrient mixture normally prescribed to cancer patients containing ascorbic acid, selenium, N-acetyl cysteine, black grapes, and other nutrients resulted in an increase of systemic availability of EGCG by 27% (405).
  • Bitter orangeBitter orange: Theoretically, bitter orange may add to the possible hypertensive effects of caffeine.
  • Caffeine containing herbsCaffeine containing herbs: Green tea is a source of caffeine. Theoretically, there may be additive side effects with other products containing caffeine.
  • CalciumCalcium: In clinical study, caffeine (400mg daily) did not affect levels of calcium absorption, endogenous fecal calcium, or urinary calcium in 16 premenopausal females (67); however, evidence from a review suggests that caffeine may increase urinary calcium levels (357).
  • Cardiovascular herbs and supplementsCardiovascular herbs and supplements: According to various reviews (358; 12; 359; 292), and animal and human studies, green tea or its constituents, may increase blood pressure, reduce LDL oxidation, inhibit platelet activation and aggregation, decrease cholesterol levels, and modify endothelial activity (360; 361; 362; 363; 364; 365; 366; 367). Caffeine acts via blockade of adenosine receptors, and theoretically, may antagonize the effects of adenosine (111).
  • Cola nutCola nut: Theoretically, caffeine from green tea may add to the effects of caffeine from cola nut.
  • ContraceptivesContraceptives: In clinical study, estrogen inhibited CYP1A2 mediated caffeine metabolism (102). Estrogen contained in oral contraceptives has been shown to lengthen the half-life of caffeine. In human study, an extended half-life was demonstratedin healthy female volunteers who used oral contraceptives for greater than three months (7.88 hours), compared to a control group (5.37 hours) (103).
  • Cytochrome P450metabolized herbs and supplementsCytochrome P450metabolized herbs and supplements: Based on animal and laboratory study, cytochrome P450 1A2 is involved in the metabolism of caffeine (373; 374; 375). According to various reviews, tea may induce cytochrome P450 enzymes 1A1, 1A2, 2B1, and 3A4 (376; 377; 378; 33). However, in one study in healthy humans, green tea extract did not alter the pharmacokinetics of two cytochrome P450 3A4 or 2D6 probe drugs, dextromethorphan and alprazolam (379). Another study also showed that green tea extract did not affect the activity of CYP1A2, CYP2D6, CYP2C9, and CYP3A4 (378). In clinical study, high doses of caffeine (400-1,000mg daily) inhibited clozapine metabolism (via CYP1A2) to an extent, which may yield clinically significant elevations in clozapine levels for some individuals (370). In human study, it was determined that among patients with cancer, green tea was included as an herbal remedy most often used that had potential interactions with the chemotherapy they were also receiving due to similar metabolism via the cytochrome P-450 metabolizing enzymes (CYPs) and/or the P-glycoprotein (P-gp) transporter (31). In human study, disulfiram consumption for at least four days was associated with a significantly increased half-life (4.1-5.7 hours, p<0.01) and decreased clearance (142-99mL per min, p<0.005) of caffeine in normal volunteers (380). Fluvoxamine is a potent inhibitor of caffeine metabolism via the cytochrome P450 system. Therefore, patients being treated with fluvoxamine may be at an increased risk of caffeine adverse effects, according to a clinical trial (381). According to secondary sources, caffeine and riluzole are both metabolized by cytochrome P450 1A2. In in vitro study, green tea extract, but not epigallocatechin gallate, significantly induced CYP1A2 mRNA expression, whereas neither CYP1A1 nor CYP3A4 mRNA expression were affected (382).
  • DiureticsDiuretics: Green tea containing caffeine may produce a diuretic effect. In clinical study, a reduction of caffeine to less than 100mg daily significantly reduced the episodes of leakage daily in patients with symptoms of urinary frequency or urge incontinence (108). Dysuria has been reported in secondary sources.
  • Ephedra (Ma huang)Ephedra (Ma huang): In theory, caffeine may increase the effects of ephedra, as evidence suggests that caffeine increases the effects of the isolated constituent, ephedrine, which is derived from Ephedra spp.
  • Fertility agentsFertility agents: Early research using a combination product containing green tea, called FertilityBlend, has been associated with some success in helping women to conceive (286). However, there is a lack of available study investigating the use of green tea alone for fertility.
  • Folic acidFolic acid: In human study, researchers noted a potential interaction between tea and folic acid, with even low concentrations (0.3g extract/250mL) of green and black tea extracts yielding decreased bioavailabilities of folic acid (406). In in vitro study, catechins and tea extracts inhibited folic acid uptake (407).
  • GuaranaGuarana: Theoretically, caffeine from green tea may add to the effects of caffeine from guarana.
  • HematologicsHematologics: In clinical study, impaired iron metabolism and microcytic anemia occurred in infants (57). In one case, green tea containing 250mg ascorbic acid per 100g tea induced transient acceleration of intravascular hemolysis in a 67 year-old man with paroxysmal nocturnal hemoglobinuria (58).
  • Hepatotoxic herbs and supplementsHepatotoxic herbs and supplements: According to various case reports and a review, hepatotoxicity (38; 39; 40; 41; 23) and acute hepatitis (42; 43) have been associated with use of green tea products.
  • Hormonal herbs and supplementsHormonal herbs and supplements: Epidemiological study suggests a relationship between green tea consumption and decreased estrogen levels in humans (60). In combination with chasteberry, as well as various vitamins, minerals, and amino acids, green tea has been associated with increased success in helping women to conceive (286). In human study, in combination with other agents (such as black cohosh, soy, and kava), green tea has been associated with relieving menopausal symptoms, including hot flashes and sleep disturbances (287).
  • Huperzine AHuperzine A: In in vitro study, EGCG enhanced the inhibitory effect of huperzine A on acetylcholinesterase (408).
  • HypertensivesHypertensives: In clinical study, green tea (81), green tea extracts (82), and specifically caffeine (83; 84; 85; 86; 87; 88; 89; 90; 91), increased systolic and diastolic blood pressure. However, there is controversy in this area and several clinical trials have not shown these effects (92; 93; 94; 95), or have demonstrated blood pressure-lowering effects (96).
  • HypoglycemicsHypoglycemics: In human study, caffeine modulated blood sugar (52); however, this was not shown in various clinical trials (150; 149; 159). Green tea extract decreased streptozotocin-induced increases in blood glucose in rats (201).
  • HypotensivesHypotensives: In clinical trials, green tea (81), green tea extracts (82), and specifically caffeine (83; 84; 85; 86; 87; 88; 89; 90; 91), increased systolic and diastolic blood pressure. However, there is controversy in this area and several clinical trials have not shown these effects (92; 93; 94; 95), or have demonstrated blood pressure-lowering effects (96).
  • ImmunosuppressantsImmunosuppressants: Based on clinical evidence, an increase in proliferation of gamma-delta T cells from subjects taking green tea, and secretion of IFN-gamma may occur (147).
  • Intraocular pressure altering herbsIntraocular pressure altering herbs: In clinical study, ingestion of caffeine increased intraocular pressure in patients with glaucoma (78). However, other clinical reports have not shown any significant effects of caffeine on intraocular pressure (79; 80).
  • IronIron: In clinical study, in thalassemia major and intermedia patients, tea caused a 41-95% inhibition of iron absorption (55). However, it has also been suggested that green tea does not inhibit iron absorption (387). Impaired iron metabolism and microcytic anemia may occur in infants of breastfeeding women consuming caffeine (56). Experts report that tea polyphenols may inhibit iron absorption by binding to iron in the gastrointestinal tract and forming insoluble complexes. This binding appears specific to non-heme forms of iron only, with black tea demonstrating greater inhibition than green tea. Ascorbic acid may overcome this binding. According to a review, it is recommended that vegetarians drink tea between meals, as binding by tea could exacerbate low availability of iron found in plant sources (59).
  • Lipoic acidLipoic acid: According to a review of cell culture and animal models, lipoic acid may combine synergistically to decrease oxidative stress, inflammation, Abeta levels, and Abeta plaque load when administered with (-)-epigallocatechin gallate (from green tea), and thus may provide a combined benefit in the treatment of Alzheimer's disease (409).
  • L-theanineL-theanine: Preliminary human research exists on the effects of L-theanine, a predominant amino acid found in green tea, indicating benefits of L-theanine in the treatment of anxiety under specific conditions (410).
  • MineralsMinerals: Serum potassium levels may decrease significantly with moderate caffeine ingestion (180 to 360mg) (411).
  • N-acetyl cysteineN-acetyl cysteine: In a crossover study, supplementation with a nutrient mixture normally prescribed to cancer patients containing ascorbic acid, selenium, N-acetyl cysteine, black grapes, and other nutrients resulted in an increase of systemic availability of EGCG by 27% (405).
  • Neurologic herbs and supplementsNeurologic herbs and supplements: In clinical study, use of green tea or constituents of green tea such as caffeine, may result in dizziness, headache, and nervous system stimulation resulting in symptoms such as delirium, confusion, restlessness, and insomnia (211; 141; 133; 175). According to clinical trials, caffeine has been found to antagonize the effects of various sedatives (98; 99; 100). Based on in vitro and animal evidence, EGCG may exert neuroprotective effects in models of Alzheimer's and Parkinson diseases (391; 392; 393)
  • P-glycoprotein modulatorsP-glycoprotein modulators: In in vitro study, green tea polyphenols inhibited p-glycoprotein activity (396; 397). The involvement of green tea in p-glycoprotein regulation has been reviewed (398; 33). In human study, it was determined that among patients with cancer, green tea was included as an herbal remedy most often used that had potential interactions with the chemotherapy they were also receiving due to similar metabolism via the cytochrome P-450 metabolizing enzymes (CYPs) and/or the P-glycoprotein (P-gp) transporter (31).
  • QuercetinQuercetin: In animal study, EGCG when administered as part of a nutrient mixture containing quercetin, increased EGCG levels in plasma vs. EGCG alone (412). The plasma C(max) was raised from 55.29±1.70 to 61.94±1.70ng/mL and 94.44±1.59ng/mL, respectively and the area under the curve over 24 hours was increased. In human study, red onion (containing quercetin) increased plasma levels of EGCG.
  • ResveratrolResveratrol: In in vitro study, resveratrol and EGCG inhibited angiogenesis in endometrial tumors (413).
  • Seizure threshold-lowering agentsSeizure threshold-lowering agents: In a case series of six depressed patients receiving electroconvulsive therapy (ECT), the effect of caffeine increased the length of the seizure and potentially enhanced the therapeutic effect (109). Also, in a review of case reports, seizures have been reported from caffeine overdose (110).
  • SeleniumSelenium: In a crossover study, supplementation with a nutrient mixture normally prescribed to cancer patients containing ascorbic acid, selenium, N-acetyl cysteine, black grapes, and other nutrients resulted in an increase of systemic availability of EGCG by 27% (405).
  • StimulantsStimulants: Theoretically, caffeine may add to effects of stimulants, such as beta adrenergic agonists. A case report indicated ischemic stroke after the nasal ingestion of a stimulant and caffeine (101).
  • Sulfotransferase 1A3 (SULT1A3) substratesSulfotransferase 1A3 (SULT1A3) substrates: In in vitro study, green tea inhibited the sulfation of dopamine and ritodine, two substrates of SULT1A3 (401).
  • TanninsTannins: In human and animal research, tannins from tea has been linked to a high rate of esophageal cancer in regions of heavy tea consumption, although other studies suggest a protective effect of green tea in relation to esophageal cancer (135; 171). Caution is advised with concomitant use of other tannin-containing herbs and supplements.
  • TobaccoTobacco: In human study, additive effects on cardiovascular parameters occurred with nicotine (91). Concomitant consumption of caffeine and cigarettes during pregnancy may place the developing fetus at higher risk for diminished growth (394).
  • VasoconstrictorsVasoconstrictors: In clinical study, both caffeine and theophylline attenuated adenosine-induced vasodilation (111).
  • VasodilatorsVasodilators: In clinical study, both caffeine and theophylline attenuated adenosine-induced vasodilation (111).
  • Vitamin KVitamin K: In human study, large amounts (one-half to one gallon) of green tea antagonized the effects of an anticoagulant, particularly Coumadin® therapy, possibly due to the small amounts of vitamin K in green tea (64; 65). Dry green tea leaves contain significantly more vitamin K than black tea leaves; green tea may contain 1,428mcg vitamin K per 100g leaf, while black tea may contain only 262mcg vitamin K per 100g leaf (66).
  • Yerba mateYerba mate: Theoretically, caffeine from green tea may add to the effects of caffeine from yerba mate.
  • Zizyphus jujubeZizyphus jujube: In in vitro study, a combination of Zizyphus jujube and green tea extracts had cytotoxic activity in HepG2 cells (414).

Green tea/Food Interactions:
  • GeneralGeneral: Food interactions associated with green tea are predominantly theoretical and based upon the adverse effect profile of caffeine.
  • Ascorbic acid containing foodsAscorbic acid containing foods: In clinical study, single doses of tea catechins decreased ascorbate levels briefly (404). In another clinical trial, however, use of a green tea extract did not affect ascorbic acid levels (294).
  • BreadBread: In human study, coingestion of the green tea extract, Polyphenon E®, with bread did not significantly modify the absorption, metabolism, and excretion of flavan-3-ols (415).
  • Caffeinecontaining foodsCaffeine-containing foods: Green tea is a source of caffeine. Theoretically, there may be additive side effects with other products containing caffeine.
  • Calcium-containing foodsCalcium-containing foods: In clinical study, caffeine (400mg daily) did not affect levels of calcium absorption, endogenous fecal calcium, or urinary calcium in premenopausal females (67); however, evidence from a review suggests that caffeine may increase urinary calcium levels (357).
  • CatechinsCatechins: Theoretically, the bioavailability of catechins, constituents of green tea, may decrease when ingested with food.
  • CheeseCheese: In human study, coingestion of the green tea extract, Polyphenon E®, with cheese did not significantly modify the absorption, metabolism, and excretion of flavan-3-ols (415).
  • Folic acid-containing foodsFolic acid-containing foods: In human study, researchers noted that a potential interaction may exist between tea and folic acid, with even low concentrations (0.3g extract/250ml) of green and black tea extracts yielding decreased bioavailabilities of folic acid (406).
  • Grapefruit juiceGrapefruit juice: Theoretically, concomitant administration of green tea with grapefruit juice may increase caffeine levels and the risk of adverse effects.
  • GrapesGrapes: In a crossover study, supplementation with a nutrient mixture normally prescribed to cancer patients containing ascorbic acid, selenium, N-acetyl cysteine, black grapes, and other nutrients resulted in an increase of systemic availability of EGCG by 27% (405).
  • Iron-containing foodsIron-containing foods: In clinical study, in thalassemia major and intermedia patients, tea inhibited iron absorption (55). However, it has also been suggested that green tea does not inhibit iron absorption (387). Impaired iron metabolism and microcytic anemia may occur in infants of breastfeeding women consuming caffeine (56). Experts report that tea polyphenols may inhibit iron absorption by binding to iron in the gastrointestinal tract and forming insoluble complexes. This binding appears specific to non-heme forms of iron only, with black tea demonstrating greater inhibition than green tea. Ascorbic acid may overcome this binding. Vegetarians are recommended to drink tea between meals, as binding by tea could exacerbate low availability of iron found in plant sources (59).
  • L-theanine-containing foodsL-theanine-containing foods: Preliminary human research exists on the effects of L-theanine, a predominant amino acid found in green tea, indicating benefits of L-theanine in the treatment of anxiety under specific conditions (410).
  • MilkMilk: Milk does not reduce polyphenol bioavailability. Polyphenols have a strong affinity for proline rich proteins (casein, milk, gelatin, saliva). However, the addition of milk to green or black tea does not appear to affect the polyphenol concentration in blood (416; 417; 328; 59). Some studies have found the tannin component to be protein bound by adding milk, thereby preventing its potential detrimental effects (416), although other studies present conflicting results (418; 328).
  • MushroomsMushrooms: Zhang et al. conducted a case-control study to examine the relationship between dietary intakes of mushrooms and green tea on the risk of breast cancer in Chinese women (419). A significant effect was observed for the combination of mushroom and green tea intake (p<0.001).
  • Potassium-containing foodsPotassium-containing foods: Serum potassium levels may decrease significantly with moderate caffeine ingestion (180 to 360mg) (411).
  • Red onionRed onion: In animal study, EGCG when administered as part of a nutrient mixture containing quercetin, increased EGCG levels in plasma vs. EGCG alone (412). The plasma C(max) was raised from 55.29±1.70 to 61.94±1.70ng/mL and 94.44±1.59ng/mL, respectively and the area under the curve over 24 hours was increased. In human study, red onion (containing quercetin) increased plasma levels of EGCG.
  • Tannin-containing foodsTannin-containing foods: There is evidence that tannins in tea is linked to a high rate of esophageal cancer in regions of heavy tea consumption, although other studies suggest a protective effect of green tea in relation to esophageal cancer (135; 171). Caution is advised with concomitant use of other tannin-containing foods.
  • Vitamin K-containing foodsVitamin K-containing foods: In a case report, large amounts (one-half to one gallon) of green tea antagonized the effects of an anticoagulant, possibly due to the small amounts of vitamin K in green tea (64; 65). Dry green tea leaves contain significantly more vitamin K than black tea leaves; green tea may contain 1,428mcg vitamin K per 100g leaf, while black tea may contain only 262mcg vitamin K per 100g leaf (66).

Green tea/Lab Interactions:
  • GeneralGeneral: Lab interactions associated with green tea are predominantly theoretical and based upon the adverse effect profile of caffeine.
  • 5-Hydroxyindoleacetic acid5-Hydroxyindoleacetic acid: According to secondary sources, caffeine may increase 5-hydroxyindoleacetic acid concentrations.
  • Antioxidant levelsAntioxidant levels: Based on clinical evidence, consumption of four cups (960mL) of green tea daily may decrease DNA damage among GSTM1-positive smokers (151). In a clinical trial, green tea, alone or in combination, increased the total antioxidant potential in hypercholesterolemic patients (154).
  • Blood pressureBlood pressure: In clinical study, green tea (81), green tea extracts (82), and specifically caffeine (83; 84; 85; 86; 87; 88; 89; 90; 91), increased systolic and diastolic blood pressure. However, there is controversy in this area and several studies have not shown these effects (92; 93; 94; 95), or have demonstrated blood pressure-lowering effects (96).
  • Body weightBody weight: In human study, there was a 0.3% reduction in body weight (0.15kg) after 12 weeks of treatment with green tea extract (not significant) (156). In other human studies, green tea extract (157; 96) and supplements (348) also demonstrated antiobesity effects, including body weight reduction.
  • CalciumCalcium: In clinical study, caffeine (400mg daily) did not affect levels of calcium absorption, endogenous fecal calcium, or urinary calcium in premenopausal females (67); however, evidence from a review suggests that caffeine may increase urinary calcium levels (357).
  • CatecholaminesCatecholamines: Caffeine overdose has been associated with elevated levels of plasma catecholamines (420). An increase in catecholamine levels was found when caffeine was administered as an adjunct to aerobic exercise (421).
  • Coagulation panelCoagulation panel: Both catechins and caffeine in green tea may exert antiplatelet activity (62; 63). In a case report, large amounts (one-half to one gallon) of green tea antagonized the effects of warfarin, possibly due to the small amounts of vitamin K in green tea (64; 65). Dry green tea leaves contain significantly more vitamin K than black tea leaves; green tea may contain 1,428mcg vitamin K per 100g leaf, while black tea may contain only 262mcg vitamin K per 100g leaf (66). Caffeine appears to inhibit the hemodynamic response to dipyridamole infusion in a dose-dependent fashion (257). According to anecdotal reports, the caffeine in green tea may prolong bleeding time. In a clinical trial, consumption of green tea did not inhibit platelet aggregation (422).
  • CreatinineCreatinine: According to secondary sources, caffeine may increase urinary creatinine concentrations. Creatinine in combination with caffeine inhibits net muscle phosphocreatine resynthesis during exercise (423).
  • ElectrolytesElectrolytes: Green tea containing caffeine may produce a diuretic effect. Serum potassium levels may decrease significantly with moderate caffeine ingestion (180 to 360mg) (411).
  • ErythropoietinErythropoietin: In preterm infants, caffeine has been found to be equivalent to theophylline in attenuating erythropoietin production (424).
  • EstrogenEstrogen: Epidemiological study suggests a relationship between green tea consumption and decreased estrogen levels in humans (60).
  • GlucoseGlucose: Based on clinical evidence, caffeine consumption may result in an increase and decrease in glucose (421). In human study, caffeine modulated blood sugar levels (52); however, this was not shown in various clinical trials (150; 149; 159). Green tea extract decreased streptozotocin-induced increases in blood glucose in rats (201).
  • HemoglobinHemoglobin: In clinical study, impaired iron metabolism and microcytic anemia occured in infants (57).
  • Hemostatic factorsHemostatic factors: In humans, coffee had no significant effect on fibrinogen, clotting factor VII activity, factor VIII antigen, protein C, and protein S (425).
  • HomocysteineHomocysteine: In human study, a dose-dependent relationship between coffee consumption and plasma homocysteine levels exists (426; 427).
  • Intraocular pressureIntraocular pressure: In clinical study, ingestion of caffeine increased intraocular pressure in patients with glaucoma (78). However, other clinical reports have not shown any significant effects of caffeine on intraocular pressure (79; 80).
  • LactateLactate: According to a clinical trial, the combination of ephedrine and caffeine may increase blood lactate levels (421).
  • Lipid profileLipid profile: In in vitro (17) and human studies (290; 291; 156; 153; 155; 292) green tea or green tea catechins exhibited antilipemic effects. A lack of effect on lipid profiles, however, has also been reported in human trials using green tea (293; 294), or caffeine alone (295; 296; 297; 298). Moderate (224mg) and high (674mg) tea catechins reduced post-prandial triglyceride levels in healthy adults (155).
  • Liver function testsLiver function tests: In human study, regular daily coffee consumption was dose-dependently linked to lower levels of liver enzymes, such as gamma-glutamyl transferase, alanine-amino transferase, and alkaline phosphatase (428). It is not certain whether this is due to caffeine, or another component in coffee, or whether this is relevant to green tea use. In a review, four studies showed a positive correlation between green tea intake and attenuation of liver disease. Moreover, a review also presented the protective tendency of green tea against liver disease (429). In a clinical trial, an increased consumption of green tea, especially more than 10 cups daily, was related to decreased concentrations of hepatological markers in serum, aspartate aminotransferase (p=0.06), alanine transferase (p=0.07), and ferritin (p=0.02), indicating that green tea may act protectively against disorders of the liver (136). According to various case reports and a review, hepatotoxicity (38; 39; 40; 41; 23) and acute hepatitis (42; 43) have been associated with use of green tea products.
  • PotassiumPotassium: Serum potassium levels may decrease significantly with moderate caffeine ingestion (180 to 360mg) (411).
  • TheophyllineTheophylline: According to a case report, intake of caffeine may cause increases in serum levels of theophylline (420).
  • Urate/uric acid (Bittner method)Urate/uric acid (Bittner method): According to anecdotal use, caffeine may cause false-positive tests. According to lab study, caffeine may falsely elevate uric acid concentrations (430).
  • UrinalysisUrinalysis: Based on human evidence, consumption of tea, which contains large amounts of ascorbic acid, may induce a false negative result during occult blood and sugar tests with urinalysis reagent strips (431). In one case, green tea containing 250mg ascorbic acid per 100g tea induced transient acceleration of intravascular hemolysis in a 67-year-old man with paroxysmal nocturnal hemoglobinuria (58).
  • Vanillylmandelic acid (VMA)Vanillylmandelic acid (VMA): According to anecdotal use, caffeine may cause small increases in VMA concentrations.

Copyright © 2011 Natural Standard (www.naturalstandard.com)

The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.
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