Green tea is generally free of side effects. The most common adverse effects reported from consuming large amounts (several cups per day) of green tea are insomnia, anxiety, and other symptoms caused by the caffeine content in the herb.

An extract of green tea taken by healthy women with a meal inhibited the absorption of non-heme iron (e.g., the form of iron in plant foods) by 26%.⁴ Frequent use of green tea could, in theory, promote the development of iron deficiency in susceptible individuals.

There have been at least 34 case reports of people developing liver damage (sometimes severe) while consuming weight-loss products that contained concentrated extracts of green tea.⁵ A cause–effect relationship was not proven, and most of the products contained other ingredients in addition to green tea extract. However, researchers have concluded that green tea extract was the probable cause of liver damage in some of the cases.⁶ Scientists have cautioned against the use of large amounts, or concentrated extracts, of green tea.

Importantly, liver damage due to green tea extract and EGCG supplementation has been reported. A review of these reports indicates high doses (500–3,000 mg green tea extract or 250–1,800 mg of EGCG taken all at once daily) used for two weeks or longer and most often on an empty stomach are more likely to cause liver injury. While individual, possibly genetic, factors may increase susceptibility, it is suggested those with pre-existing liver disease and obesity may be at greatest risk.⁷

In addition, there is a case report in which a person developed thrombotic thrombocytopenic purpura (a condition in which a bruising develops as a result of a low platelet count) after consuming a weight-loss product that contained green tea extract for 2 months. Green tea was not proven to be the cause of this problem.⁸