Green Tea

Purported Benefits, Side Effects & More
Green Tea

Common Names

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

For Patients & Caregivers

Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.

What is it?

Green tea is a drink made from the Camellia sinensis plant. Fresh leaves are cut from this plant and steamed to make the tea.

You can drink green tea or take it orally (by mouth) in a pill or tablet as a dietary supplement. You can also use green tea extract on your skin as a cream.

What are the potential uses and benefits?

Green tea is used to:

  • Lower cholesterol levels
  • Prevent heart disease
  • Help treat high blood pressure
  • Prevent tooth decay
  • Treat warts on your genitals or anus

Green tea also has other uses that haven’t been studied by doctors to see if they work.

It’s generally safe to drink green tea and use it in food. Talk with your healthcare providers before taking green tea supplements. Herbal supplements are stronger than the herbs you would use in cooking.

Supplements can also interact with some medications and affect how they work. For more information, read the “What else do I need to know?” section below.

What are the side effects?

Side effects of taking green tea supplements may include:

  • Nausea (feeling like you’re going to throw up)
  • Stomach pain
  • Drinking too much green tea can disrupt your sleep, and can also give you headaches
What else do I need to know?
  • Talk to your doctor if you’re taking iron supplements. Green tea can affect the way iron is absorbed in the body. 
  • Don’t drink green tea if you’re pregnant or breastfeeding. Green tea has caffeine in it that may cause your baby to have trouble sleeping.
  • Avoid drinking green tea if you have a stomach ulcer (sore in the lining of your stomach). Green tea may make your ulcer worse.
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For Healthcare Professionals

Scientific Name
Camellia sinensis
Clinical Summary

Green tea is a beverage derived from the unfermented leaves of a plant native to Asia. Green tea extract is marketed as a dietary supplement to regulate blood sugar, cholesterol, blood pressure, and for weight loss and cancer prevention. Active constituents include the polyphenol epigallocatechin-3-gallate (EGCG), caffeine, and theanine. Green tea consumption may reduce the risk of dementia (90), hypertension (9) and cardiovascular disease (14), and a theaflavin-enriched extract was shown to decrease the levels of low-density lipoprotein cholesterol (LDL-C) (8). Some findings suggest that green tea can enhance glucose tolerance in healthy individuals (11) (12), and reduce fasting blood glucose levels (78) whereas data on obese (10) (60) and diabetic patients (13) are mixed. A green tea extract was shown useful, as an adjunct to the standard therapy, to improve arterial stiffness in patients with type-2 diabetes (80). Green tea consumption has also been associated with reduced risk of death from all causes and cardiovascular tea but not cancer (91); and with a reduction in self-reported depression (77) although conflicting data suggest no such benefit (92).

Topical application of green tea extracts was found effective against external genital and perianal warts (15) (16) (17). One such extract, sinecatechin, is FDA-approved for this use. Green tea extracts were also found beneficial for the treatment of acne  (83).

Investigations into the anticancer potential of green tea and its active compounds showed preventive benefits in patients with oral premalignant lesions (41), and in those at high risk of developing liver (61) or colorectal cancers (64). Another extract produced beneficial biologic responses in patients with chronic lymphocytic leukemia (52).

In addition, topical EGCG was shown to help relieve radiation-induced dermatitis in breast cancer patients (74) (75) while oral EGCG was found effective against acute radiation-induced esophagitis in lung cancer patients (76). But EGCG-enriched green tea was not useful as maintenance intervention in women with advanced stage ovarian cancer after standard treatment (71), and EGCG supplementation did not reduce the likelihood of prostate cancer (63).

In prostate cancer patients, supplementation with a blend of green tea, pomegranate, broccoli, and curcumin had a protective effect following post-radical treatment (59). Green tea consumption also decreased PSA levels in patients prior to prostatectomy (72). However, higher doses of green tea catechins were associated with elevated incidence of prostate cancer in men with high grade prostatic intraepithelial neoplasia and/or atypical small acinar proliferation (73).

Furthermore, green tea extract combined with curcumin may be beneficial in the treatment of potentially malignant oral disorders (84). Data on whether green tea can reduce the risk of gastric and esophageal cancers are mixed (5) (6) (42); and there was no association between green tea and the risk of developing ovarian cancer  (93), malignant lymphoma or multiple myeloma (65), although data indicate that tea intake may reduce the risk of myelodysplastic syndrome (56). Other findings suggest consumption may actually increase breast cancer risk in postmenopausal women (62). Future studies are needed to resolve this ambiguity and to determine potential beneficial effects of green tea consumption on overall cancer risk (81).

Moderate consumption of green tea was not associated with increased risk of kidney stone formation (85). But the caffeinated form may cause insomnia and nausea. Therefore, decaffeinated products may be preferred due to lower incidence of adverse events, although data are inconsistent regarding the relative efficacy of caffeinated versus decaffeinated teas. Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity (49). In humans, EGCG intake (200 mg, twice daily for one year) was shown to be safe (70) based on clinical data, whereas a daily dose amounting to 800 mg EGCG was associated with elevated liver enzymes, which was reversible with cessation of consumption (67). It is important to distinguish food from concentrated supplements because the dosage varies considerably, and that in turn can either confer benefits or cause harmful effects.

Purported Uses and Benefits
  • High cholesterol
  • Cardiovascular disease
  • Hypertension
  • Cancer prevention
  • Oral health
  • Genital warts
Mechanism of Action

Green tea is thought to confer cardiovascular protection by increasing HDL cholesterol and lowering LDL cholesterol and triglycerides (8) (29), as well as by blocking platelet aggregation. The tannins may have antibacterial properties (28) and can produce antidiarrheal effects. The flavonoid constituents may reduce lipoprotein oxidation (30).

Modulation of blood pressure by green tea is thought to be mediated through EGCG. It induces nitric oxide (NO) production via activation of endothelial NO synthase, resulting in vasodilation (27). In healthy subjects, a green tea extract modulated effective brain connectivity during working memory processing (55). EGCG inhibited aggregation of tau protein, thereby reducing toxicity in neuronal model cells (57).

Anticancer activities have been related to polyphenol content, with chemopreventive properties attributed to EGCG via apoptotic induction and tumor antiangiogenesis (20). EGCG may inhibit enzymes involved in cell replication and DNA synthesis by interfering with cell-to-cell adhesion or 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). It also inhibited DNA replication in leukemia cancer cell lines (24), and modulated vascular endothelial growth factor (VEGF) leading to apoptosis in leukemic cells (7).

Administration of green tea inhibits UVB light-induced carcinogenesis; when given before and during carcinogen treatment, it reduced incidence and number of stomach and esophageal tumors in mice (25).

  • 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 should avoid drinking because it can stimulate the production of gastric acid (29).
  • The United States Pharmacopeia included this cautionary labeling requirement in its Powdered Decaffeinated Green Tea Extract monograph: “Do not take on an empty stomach. Take with food. Do not use if you have a liver problem and discontinue use and consult a healthcare practitioner if you develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice (yellowing of the skin or eyes)” (86).
Adverse Reactions
  • Green tea is generally considered safe, but because of its caffeine content, excessive consumption can disrupt sleep and cause headaches. A few cases of hepatotoxicity have also been associated with green tea extracts containing high EGCG levels (44) (46) (47) (58) (94).
  • Pruritic swelling and darkening of lower lip: In a 40-year-old woman following consumption of excessive green tea over several years. Her symptoms resolved after discontinuing green tea (45).
  • Thrombotic thrombocytopenic purpura: In a 38-year-old woman after taking a green tea supplement for weight loss. Her symptoms resolved after treatment (48).
  • Green tea-induced anaphylaxis: In a 9-year-old boy after consuming matcha tea-flavored ice cream. His symptoms improved after treatment (95).
  •  Clinical trials reported the following adverse reactions that appear to be dose dependent:
    — Nausea, abdominal pain, and transaminitis after intake of high dose EGCG (2000 mg orally twice per day) in patients with early stage chronic lymphocytic leukemia (52)
    — Elevation in alanine aminotransferase (ALT) levels, also after consumption of high amounts of EGCG (843 mg daily for one year) by postmenopausal women at risk for breast cancer (68)
    — In another study of EGCG in breast cancer patients, rectal bleeding (with 800mg daily), weight gain, indigestion and insomnia (daily dose of 1200 mg), and liver function abnormality (at 1,600 mg daily) were reported (69)
    However, in a study of men with high-grade prostatic intraepithelial neoplasia (HGPIN) and/or atypical small acinar proliferation (ASAP), EGCG (200 mg, twice daily for one year) was shown to be safe (70).
  • When consumed during fasting, green tea extract increased the risk of toxicity in animal studies. Whether it has the same effect in humans is not known (49).
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 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) (87).
  • 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).
  • Palbociclib: The bioavailability of Palbociclib decreased following administration of green tea extract in a murine model (79). Clinical relevance has yet to be determined.
  • Ticagrelor: Green tea polyphenols were reported to inhibit the metabolism of ticagrelor, in vitro. Clinical relevance has yet to be determined (82).
  • Atorvastatin: Green tea limited hepatic drug uptake and increased plasma exposure to atorvastatin in a murine model. Clinical significance is not known (88).
  • Rosuvastatin: Repeated administration of green tea extract significantly lowered the systemic exposure of rosuvastatin in healthy volunteers  (96).
  • Ziprasidone: A 23-year-old man who was stable for several months on this medication became psychotic within days following use of a green tea extract for weight loss. His symptoms resolved after eliminating green tea  (89).
Herb Lab Interactions
  • Caffeine in green tea may increase PT / PTT
Dosage (OneMSK Only)
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