Green Tea

Common Names

  • Chinese tea
  • Green tea extract
  • Green tea polyphenols
  • Epigallocatechin gallate (EGCG)

For Patients & Caregivers

How It Works

Green tea may help lower cholesterol. Evidence of its cancer preventive effects in humans is not conclusive.

Green tea contains substances called polyphenols, which scientists think contribute to its anti-cancer activity. Laboratory studies of one polyphenol, catechin epigallocatechin-3-gallate (EGCG), show that it may interfere with several of the processes involved in cell replication, causing tumor cell death (apoptosis). It also might slow the formation of blood vessels around tumors. Epigallocatechin (ECG), another polyphenol, stops leukemic cells from multiplying in laboratory studies. As a proven antioxidant, green tea may repair cell damage, but whether it can prevent cancer is uncertain. It is also unknown how it might help protect the heart, but it reduces LDL (“bad”) cholesterol and increases HDL (“good”) cholesterol. Tannins present in green tea generally have antibacterial properties.

Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity. Human studies are needed.

Purported Uses

  • As an antioxidant
    There is mixed evidence.
  • To prevent and treat cancer
    Although laboratory studies show an anti-cancer effect, a few clinical trials and population surveys show mixed results. A number of studies in China have suggested that high intake may protect against cancers of the colon and stomach.
  • To lower cholesterol and prevent heart disease
    One clinical trial showed that a theaflavin-enriched green tea extract can be used with other dietary approaches to lower certain type of cholesterol. Other studies report no effects on lipoprotein oxidation (contributes to atherosclerosis).
  • To improve mental functioning; for clear thinking
    Caffeinated form may stimulate the nervous system.
  • To lower high blood pressure
    Studies show a reduction in the risk of developing high blood pressure.
  • To prevent tooth decay
    Laboratory studies show that green tea may prevent bacteria from attaching to teeth. This use has not been tested in clinical trials.
  • To lose weight
    One human study shows increased expenditure of energy.
  • To increase water loss (as a diuretic)
    No studies support this use.

Patient Warnings

  • Infants should not be fed green tea, as it may interfere with iron metabolism and cause anemia.

Do Not Take If

  • You are pregnant or breastfeeding: Caffeine passes into breast milk and may cause insomnia in the infant.
  • You have a peptic ulcer: Green tea stimulates the production of gastric acid.
  • You are taking adenosine: Caffeine may lessen its effects.
  • You are taking atropine: Green tea may reduce its absorption from the gut and therefore lessen its effects.
  • You are taking codeine: Green tea may reduce its absorption from the gut and therefore lessen its effects.
  • You take warfarin or other blood thinners: In theory, very large amounts on the order of one-half to one gallon per day might lessen the effect of these drugs.
  • You are using bortezomib (Velcade®): Polyphenols can inhibit the effects of this drug.
  • You are taking drugs that are substrates of cytochrome P450 3A4: Green tea may increase the risk of side effects of these drugs.
  • You are taking drugs that are substrates of UGT (Uridine 5’-diphospho-glucuronosyltransferase) enzymes: Green tea may increase the risk of side effects of these drugs.
  • You are taking acetaminophen: Green tea increased liver injury caused by acetaminophen in mice when it was given after acetaminophen.

Side Effects

  • Nausea, stomach upset
  • Caffeine can cause insomnia, nervousness, or irritability.

Case Reports

  • Liver toxicity: Several cases have been associated with consumption of green tea.
  • Swelling, itchiness, and darkening of lower lip: In a 40-year-old woman following use of green tea for several years.
  • Blood clots in small blood vessels: In a 38-year-old woman following green tea supplementation for weight loss.

Special Point

  • Green tea can reduce the absorption of iron from the gut. It should be taken either 2 hours before or 4 hours after taking an iron supplement.
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For Healthcare Professionals

Scientific Name

Camellia sinensis

Clinical Summary

Green tea, a common beverage consumed in Asia, ha been used to prevent and treat hyperlipidemia, hypertension, atherosclerosis and cancer. The active constituent is epigallocatechin-3-gallate (EGCG), which accounts for 40% of the total polyphenol content. Regular consumption of may reduce the risk of hypertension (9) and positively affect mood (10). It may also enhance glucose tolerance in healthy individuals (11) (12), but does not improve insulin sensitivity or glycemic control in overweight or obese males (10) or in individuals with type II diabetes (13). A decaffeinated extract was not associated with overall reductions in adiposity or improvements in BMD in overweight/obese postmenopausal women, but may reduce tissue and gynoid fat in those with higher BMI (60). It may also reduce mortality due to cardiovascular disease in both men and women (14) and a theaflavin-enriched extract lowered the low-density lipoprotein cholesterol (LDL-C) level (8).

Topical application of green tea extracts are effective against external genital and perianal warts (15) (16) (17). One such extract, sinecatechin, is an FDA-approved drug indicated for this use.

Studies of chemopreventive activity of green tea indicated positive results (1) (2) (3) (4) (5) (41). EGCG has been shown to modulate vascular endothelial growth factor (VEGF) leading to apoptosis in leukemic cells (7) and a green tea extract may benefit patients with chronic lymphocytic leukemia (52). In a population at high-risk for hepatocellular carcinoma, green tea polyphenols were found to mitigate fumonisin biomarkers (61). Data are not conclusive on whether consumption of green tea reduces the risk of colorectal and stomach cancers in women (6) (42); another finding suggests that consumption may increase the risk of breast cancer in postmenopausal women (62). In other studies it was shown to reduce the risk of myelodysplastic syndromes (56) ; and supplementation with a blend of green tea, pomegranate, broccoli and curcumin resulted in a reduction in the rate of prostate-specific antigen (PSA) increase among men with prostate cancer following a PSA relapse post-radical treatment (59). However, supplementation with EGCG did not reduce the likelihood of prostate cancer (63).

The caffeinated form may cause insomnia and nausea. Use of decaffeinated products may be preferred due to lower incidence of adverse events, but data are inconsistent regarding the relative efficacy of caffeinated versus decaffeinated teas.  Patients undergoing chemotherapy should avoid consumption. Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity (49). Human studies are warranted.

Purported Uses

  • Cancer prevention
  • Cancer treatment
  • Cardiovascular disease
  • Cognitive improvement
  • GI disorders
  • Hypertension
  • Weight loss

Mechanism of Action

 Green tea is thought to confer cardiovascular protection by increasing HDL cholesterol, decreasing LDL cholesterol and triglycerides (8) (29), as well as by blocking platelet aggregation. The tannins may have antibacterial properties (28) and can produce anti-diarrheal effects. The flavonoid constituents may reduce lipoprotein oxidation (30). Caffeine has stimulatory effects and is responsible for the majority of adverse effects and drug interactions. It is unknown whether removing caffeine alters green tea’s activities (25).

The mechanism by which green tea influences blood pressure is thought to be mediated through the catechin, epigallocatechin-3-gallate’s (EGCG) modulation of vascular constriction. EGCG induces nitric oxide (NO) production through the activation of endothelial NO synthase, resulting in vasodilation (27). In a recent study, green tea extract was shown to modulate effective brain connectivity during working memory processing in healthy subjects (55). EGCG was found to inhibit aggregation of tau protein, thereby reducing toxicity in neuronal model cells (57).

The anticancer activity is thought to be related to polyphenol content with the chemopreventive attributes associated with EGCG, which is thought to induce apoptosis and tumor antiangiogenesis (20). EGCG may inhibit enzymes involved in cell replication and DNA synthesis by interfering with cell-to-cell adhesion or via inhibition of intracellular communication pathways required for cell division (21). In vitro data indicate that concentrations of 30 mcg/mL EGCG and (-)-epigallocatechin (EGC) inhibit lipoxygenase-dependent arachidonic acid metabolism by 30-75% in normal human colon mucosa and colon cancers (22). Other studies in human colon cancer cell lines suggest that EGCG inhibits topoisomerase I, but not topoisomerase II (23). EGCG also inhibits DNA replication in vitro in leukemia cancer cell lines (24). EGCG has been shown to modulate vascular endothelial growth factor (VEGF) leading to apoptosis in leukemic cells (7).

Administration of green tea inhibits UVB light-induced carcinogenesis (25), and when given before and during carcinogen treatment, reduces the incidence and number of stomach and esophageal tumors in mice (26). Topical EGCG may be useful as chemoprevention for skin cancer, but additional research and formulation are necessary (3).


  • Although the U.S. Food and Drug Administration (FDA) includes tea on their list of “Generally Recognized As Safe” substances, pregnant women and women who breastfeed should limit their intake because of caffeine content.
  • Because tea can pass into breast milk, it may cause sleep disorders in nursing infants. Ingestion by infants has been linked to impaired iron metabolism and microcystic anemia.
  • Individuals with peptic ulcers sh avoid drinking because it can stimulate the production of gastric acid (29).

Adverse Reactions

  • Nausea and GI upset, possibly due to tannin content. Insomnia, irritability, and nervousness can occur due to caffeine content.

Case Reports

  • Hepatitis: Several cases have been associated with consumption (44) (46) (47) (58).
  • Pruritic swelling/darkening of lower lip: In a 40-year-old woman following consumption for several years (45).
  • Thrombotic thrombocytopenic purpura: In a 38-year-old woman following supplementation for weight loss (48).

Herb-Drug Interactions

  • Adenosine: The caffeine content may inhibit the hemodynamic effects of adenosine (18).
  • Anticoagulants / Antiplatelets: Theoretically, consumption of large amounts (.5-1 gallon/day) may provide enough vitamin K to antagonize the effects of anticoagulants and antiplatelet agents, though this effect has not been reported in humans (29) (34).
  • Atropine: The tannin content may reduce the absorption of atropine.
  • Iron supplements: The tannin content in may reduce the bioavailability of iron. Green tea should be taken either 2 hours before or 4 hours following iron administration.
  • Codeine: The tannin content may reduce the absorption of codeine (18).
  • Bortezomib: EGCG and other polyphenols can inhibit the therapeutic effect of bortezomib (Velcade®) and other boronic acid based proteasome inhibitors (37).
  • Tamoxifen: EGCG was shown to increase the oral bioavailability of tamoxifen, increasing the potential for their interactions (38).
  • Verapamil: The bioavailability of Verapamil increased significantly in the presence of EGCG, thought to be due to P-glycoprotein inhibition by EGCG (39).
  • Irinotecan: A study found EGCG to inhibit transport of irinotecan and its metabolite SN-38 into biliary elimination, resulting in their prolonged half-life which can increase toxicity (40).
  • Cytochrome P450 3A4 substrates: Green tea extract inhibits CYP 3A4 enzyme and can affect the intracellular concentration of drugs metabolized by this enzyme (43) (44).
  • UGT (Uridine 5’-diphospho-glucuronosyltransferase) substrates: Green tea modulates UGT enzymes in vitro and can increase the side effects of drugs metabolized by them (51).
  • Acetaminophen: Green tea was shown to increase acetaminophen-induced hepatotoxicity in mice when administered following acetaminophen  (53).
  • Nadolol: Green tea extract inhibits OATP1A2 transporter and can reduce the absorption and plasma concentration of substrate drugs, like nadolol (54).

Herb Lab Interactions

  • Caffeine in green tea may increase PT / PTT.

Dosage (OneMSK Only)


  1. Hsu SD, et al. Chemoprevention of oral cancer by green tea. Gen Dent 2002;50:140-6.

  2. Pisters KM, et al. Phase I trial of oral green tea extract in adult patients with solid tumors. J Clin Oncol 2001;19:1830-8.

  3. Nechuta S, Shu XO, Li HL, et al. Prospective cohort study of tea consumption and risk of digestive system cancers: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 2012 Nov;96(5):1056-63. doi: 10.3945/ajcn.111.031419. Epub 2012 Oct 10.

  4. Yang YC, et al.The protective effect of habitual tea consumption on hypertension. Arch Intern Med. 2004 Jul 26;164(14):1534-40.

  5. Tsuneki H, Ishizuka M, Terasawa M, Wu JB, Sasaoka T, Kimura I. Effect of green tea on blood glucose levels and serum proteomic patterns in diabetic (db/db) mice and on glucose metabolism in healthy humans.BMC Pharmacol. Aug 26 2004;4:18.

  6. Venables MC, Hulston CJ, Cox HR, Jeukendrup AE. Green tea extract ingestion, fat oxidation, and glucose tolerance in healthy humans. Am J Clin Nutr. Mar 2008;87(3):778-784.

  7. Stockfleth E, Beti H, Orasan R, et al. Topical Polyphenon E in the treatment of external genital and perianal warts: a randomized controlled trial.Br J Dermatol. Jun 2008;158(6):1329-1338.

  8. Tatti S, Swinehart JM, Thielert C, Tawfik H, Mescheder A, Beutner KR. Sinecatechins, a defined green tea extract, in the treatment of external anogenital warts: a randomized controlled trial. Obstet Gynecol. Jun 2008;111(6):1371-1379.

  9. Brinker F. Herb Contraindications and Drug Interactions, 3rd ed. Sandy (OR): Eclectic Medical Publications; 2001.

  10. Yang CS, et al. Prevention of carcinogenesis by tea polyphenols. Drug Metab Rev 2001;33:237-53.

  11. Hamilton-Miller JM. Anti-cariogenic properties of tea (Camellia sinensis). J Med Microbiol 2001;50:299-302.

  12. LaValle JB, et al. IN: Natural Therapeutics Pocket Guide 2000-2001;452-4.

  13. van het Hof KH, et al. Bioavailability of catechins from tea: the effect of milk. Eur J Clin Nutr 1998;52:356-9.

  14. Taylor JR, Wilt VM. Probable antagonism of warfarin by green tea. Ann Pharmacother 1999;33:426-8.

  15. Tsubono Y, et al. Green tea and the risk of gastric cancer in Japan. N Engl J Med 2001;344:632-6

  16. Li Q, Kakizaki M, Kuriyama S, et al. Green tea consumption and lung cancer risk: the Ohsaki study. Br J Cancer 2008 Oct 7;99(7):1179-84.

  17. Chung JH, Choi DH, Choi JS. Effects of oral epigallocatechin gallate on the oral pharmacokinetics of verapamil in rats. Biopharm Drug Dispos. 2009 Mar;30(2):90-3.

  18. Tsao AS, Liu D, Martin J, et al. Phase II randomized, placebo-controlled trial of green tea extract in patients with high-risk oral premalignant lesions. Cancer Prev Res (Phila Pa). 2009 Nov;2(11):931-41.

  19. Myung SK, Bae WK, Oh SM, et al. Green tea consumption and risk of stomach cancer: a meta-analysis of epidemiologic studies. Int J Cancer. 2009 Feb 1;124(3):670-7.

  20. Wanwimolruk S, Wong K, Wanwimolruk P. Variable inhibitory effect of different brands of commercial herbal supplements on human cytochrome P-450 CYP3A4. Drug Metabol Drug Interact. 2009;24(1):17-35.

  21. Mazzanti G, Menniti-Ippolito F, Moro PA, et al. Hepatotoxicity from green tea: a review of the literature and two unpublished cases. Eur J Clin Pharmacol. 2009 Apr;65(4):331-41.

  22. Lee JI, Cho BK, Ock SM, Park HJ. Pigmented contact cheilitis: from green tea? Contact Dermatitis. 2010 Jan;62(1):60-1.

  23. Vanstraelen S, Rahier J, Geubel AP. Jaundice as a misadventure of a green tea (camellia sinensis) lover : a case report. Acta Gastroenterol Belg. 2008 Oct-Dec;71(4):409-12.

  24. Verhelst X, Burvenich P, Van Sassenbroeck D, Gabriel C, Lootens M, Baert D. Acute hepatitis after treatment for hair loss with oral green tea extracts (Camellia Sinensis). Acta Gastroenterol Belg. 2009 Apr-Jun;72(2):262-4.

  25. Liatsos GD, Moulakakis A, Ketikoglou I, Klonari S. Possible green tea-induced thrombotic thrombocytopenic purpura. Am J Health Syst Pharm. 2010 Apr 1;67(7):531-4.

  26. Engdal S, Nilsen OG. In vitro inhibition of CYP3A4 by herbal remedies frequently used by cancer patients. Phytother Res. 2009 Jul;23(7):906-12.

  27. Shanafelt TD, Call TG, Zent CS, et al. Phase 2 trial of daily, oral polyphenon E in patients with asymptomatic, Rai stage 0 to II chronic lymphocytic leukemia. Cancer. 2012 Jul 3. doi: 10.1002/cncr.27719. [Epub ahead of print]

  28. Salminen WF, Yang X, Shi Q, Greenhaw J, Davis K, Ali AA. Green tea extract can potentiate acetaminophen-induced hepatotoxicity in mice. Food Chem Toxicol. 2012 May;50(5):1439-46.

  29. S Misaka, J Yatabe, F Müller, et al. Green tea ingestion greatly reduces plasma concentrations of nadolol in healthy subjects. Clin Pharmacol Ther. 2014 Apr;95(4):432-8.

  30. Schmidt A, Hammann F, Wölnerhanssen B, et al. Green tea extract enhances parieto-frontal connectivity during working memory processing. Psychopharmacology (Berl). 2014 Oct;231(19):3879-88.

  31. Liu P, Zhang M, Jin J, Holman CD. Tea consumption reduces the risk of de novo myelodysplastic syndromes. Leuk Res. 2015 Feb;39(2):164-9.

  32. Fernández J, Navascués C, Albines G, Franco L, Pipa M, Rodríguez M. Three cases of liver toxicity with a dietary supplement intended to stop hair loss. Rev Esp Enferm Dig.2014 Dec;106(8):552-5.

  33. Kumar NB, Pow-Sang J, Egan KM, et al. Randomized, Placebo-Controlled Trial of Green Tea Catechins for Prostate Cancer Prevention. Cancer Prev Res (Phila).2015 Oct;8(10):879-87.

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