Green tea is generally free of side effects. The most common adverse effects reported from consuming large amounts (a few portions 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-haem iron (e.g., the form of iron in plant foods) by 26%.³⁴ 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.³⁵ 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.³⁶ Scientists have cautioned against the use of large amounts, or concentrated extracts, of green tea. 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.³⁷