Green Tea

Green Tea

Green Tea

Common Names

  • Chinese tea
  • green tea extract
  • green tea polyphenols
  • epigallocatechin gallate (EGCG)

For Patients & Caregivers

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. Tannins like those found in green tea generally have antibacterial properties. It is unknown how green tea might help protect the heart, but it reduces LDL (“bad”) cholesterol and increases HDL (“good”) cholesterol.

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

  • As an antioxidant
    Studies have yielded 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 of green tea may protect against cancers of the colon and stomach.
  • To lower cholesterol and prevent heart disease
    One clinical trial showed that theaflavin-enriched green tea extract can be used with other dietary approaches to lower certain type of cholesterol. Other studies have generally shown that green tea has no effect lipoprotein oxidation (a contributing factor in atherosclerosis).
  • To improve mental functioning; for clear thinking
    Caffeinated green tea may stimulate the nervous system.
  • To lower high blood pressure
    There is evidence that green tea can reduce 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 supports that green tea increases energy expenditure.
  • To increase water loss (as a diuretic)
    No studies support this use, although green tea contains caffeine, a diuretic.
  • Infants should not be fed green tea (it may interfere with iron metabolism and cause anemia).
  • You are pregnant or breast-feeding (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 of green tea, one half to one gallon/day, might lessen the effect of these drugs).
  • You are using bortezomib (Velcade®) (Polyphenols in green tea can inhibit the effect 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).
  • Nausea
  • Stomach upset
  • The caffeine in green tea can cause insomnia, nervousness, or irritability.
  • Liver toxicity has been associated with green tea.
  • Reported: A 40-year-old woman developed pruritic swelling and darkening of lower lip following use of green tea for several years. Symptoms resolved after discontinuing consumption of green tea.
  • Reported: A 38-year-old woman developed thrombotic thrombocytopenic purpura following green tea supplementation for weight loss.
  • Green tea can reduce the absorption of iron from the gut. If you take iron supplements, green tea should be taken either two hours before or four hours after taking an iron supplement.
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For Healthcare Professionals

Camellia sinensis

Green tea is a common beverage consumed in Asia. In recent years, green tea and its extracts have 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 of green tea extract. Regular consumption of green tea 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). Green tea may reduce mortality due to cardiovascular disease in both men and women (14). A Theaflavin-enriched green tea extract can be used to lower low-density lipoprotein cholesterol (LDL-C) (8).

Topical application of green tea extracts effectively treats external genital and perianal warts (15) (16) (17). A green tea extract, sinecatechin, is an FDA approved drug.

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). Consumption of green tea is associated with reduced risk of colorectal and stomach cancers in women (6). However, a meta analysis found no such benefits (42).
Another study suggests an association between green tea intake and reduced risk of myelodysplastic syndromes (56).

Caffeinated green tea 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. Tannins in green tea may reduce absorption and bioavailability of codeine, atropine, and iron supplements (18). The polyphenolic constituents in green tea can negate the therapeutic effect of bortezomib, an anticancer drug (37). Patients undergoing chemotherapy should avoid consuming green tea products.

Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity (49). Human studies are warranted.

Green tea

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

The tannins in green tea may have antibacterial properties (28) and can produce anti-diarrheal effects. 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. Flavonoids present in green tea may reduce lipoprotein oxidation (30). Green tea also contains caffeine, which 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 of green tea is thought to be related to its polyphenol content. Its chemopreventive attributes are 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 breast feed should limit their intake of green tea because of caffeine content.
Because tea can pass into breast milk, it may cause sleep disorders in nursing infants. Green tea ingestion in infants has been linked to impaired iron metabolism and microcytic anemia.
Individuals with peptic ulcers may want to avoid drinking green tea because it can stimulate the production of gastric acid. (29)

Case Reports:

  • Nausea and GI upset, possibly due to tannin content. Insomnia, irritability, and nervousness can occur due to caffeine content.
  • Several cases of hepatitis have been associated with consumption of green tea (44) (46) (47) (58).
  • A 40-year-old woman developed pruritic swelling and darkening of lower lip following use of green tea for several years. Symptoms resolved after discontinuing consumption of green tea (45).
  • A 38-year-old woman developed thrombotic thrombocytopenic purpura following green tea supplementation for weight loss (48).
  • Adenosine: The caffeine content in green tea may inhibit the hemodynamic effects of adenosine (18).
  • Anticoagulants / Antiplatelets: Theoretically, consumption of large amounts of green tea (.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 in green tea may reduce the absorption of atropine.
  • Iron supplements: The tannin content in green tea 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 in green tea may reduce the absorption of codeine (18).
  • Bortezomib: EGCG and other polyphenols in green tea 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).

Caffeine in green tea may increase PT / PTT.


  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.

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