Green tea is a beverage derived from the unfermented leaves of a plant native to Asia. Green tea extract is marketed as an antioxidant and dietary supplement to support cardiovascular, metabolic, cognitive, and cellular health. Active constituents include the polyphenol epigallocatechin-3-gallate (EGCG), caffeine, and theanine.

Green tea consumption may reduce risk for dementia ⁽¹⁾, hypertension ⁽²⁾, cardiovascular disease ⁽⁶³⁾, small vessel stroke ⁽⁷¹⁾, and all-cause but not cancer-related mortality ^(3) (4). Data are mixed on whether it can reduce risk of gastric and esophageal cancers ^{(5) (6) (7)}. Other findings suggest preventive benefits with green tea and its active compounds against oral premalignant lesions ⁽⁸⁾ and for those at high risk of developing liver or colorectal cancers ^(9) (10) but not colorectal adenomas ⁽⁶⁴⁾. There is no association between green tea and risk for malignant lymphoma or multiple myeloma ⁽¹¹⁾, but intake may reduce risk for myelodysplastic syndrome ⁽¹²⁾ and kidney cancer ⁽⁶⁵⁾.

In prostate cancer patients, a blend of green tea, pomegranate, broccoli, and curcumin had a protective effect following post-radical treatment ⁽¹³⁾. Green tea consumption also decreased PSA levels in patients prior to prostatectomy ⁽¹⁴⁾. However, combination supplementation with the highest nontoxic doses of green tea catechins, selenium, and lycopene was associated with an elevated incidence of prostate cancer in high-risk patients ⁽¹⁵⁾, and long-term intake of an EGCG product did not reduce prostate cancer risk ⁽¹⁶⁾.

Other limited data suggest oral EGCG may induce a biologic response in patients with chronic lymphocytic leukemia ⁽¹⁷⁾ or reduce radiation-induced esophagitis in lung cancer patients ⁽¹⁸⁾, but did not prevent recurrence in advanced ovarian cancer ⁽¹⁹⁾. Topical EGCG may help relieve radiation-induced dermatitis in breast cancer patients ^(20) (21), but these results are also preliminary.    

Green tea reduced the risk of recurrence in breast cancer patients  ⁽⁷²⁾, but regular consumption may elevate the risk in some postmenopausal women ⁽²²⁾. Future studies are needed to resolve this ambiguity and to determine whether there are potential benefits on overall cancer risk ⁽²³⁾.

Additional studies suggest positive effects of green tea on lipid profiles ⁽⁷³⁾ and glycemic control in type-2 diabetes patients ⁽⁶⁶⁾, for weight reduction in  obese/overweight adults ⁽⁷⁴⁾ and in women with polycystic ovary syndrome ⁽⁶⁷⁾, for sarcopenia in older adults ⁽⁷⁵⁾, and for patients with acne ⁽⁷⁶⁾.

Moderate green tea consumption was not associated with increased risk of kidney stone formation ⁽²⁴⁾, but the caffeinated form may cause insomnia and nausea. Animal models suggest use of green tea extract during fasting may increase toxicity risk ⁽²⁵⁾. Clinical data show that EGCG 200 mg twice daily for one year is safe ⁽²⁶⁾, whereas 800 mg daily was associated with elevated liver enzymes ⁽²⁷⁾.