Does green tea protect against cancer?
Current research provides no convincing evidence that green tea prevents cancer. Drinking a few cups a day is perfectly fine, but do not buy concentrated supplements in the hope of cancer prevention.
In laboratory and animal research, catechins from green tea -- particularly the compound EGCG -- inhibit the growth and spread of tumour cells through multiple pathways. That is an interesting biological signal, but whether it also offers protection in humans is unknown. Laboratory results do not translate into a clinical effect anywhere near as often as one might hope.
In humans, the picture is thin and contradictory. The largest summary of available research (more than 1.1 million participants) found a slightly reduced overall cancer risk among heavy green tea drinkers, but this difference was not statistically convincing. On cancer mortality, green tea had virtually no demonstrable effect. For specific cancer types such as oesophageal, prostate and bladder cancer, cohort studies sometimes pointed to an increased risk, while other study types found a reduction or nothing at all. For stomach cancer, 17 studies produced barely any consistent result.
Green tea extracts taken as a supplement show even more uncertainty. Small trials suggested a possible reduction in prostate cancer, but the margins are so wide that no conclusions can be drawn. More worryingly, two trials in women found an increased risk of gynaecological cancer with the use of an extract supplement. This too was statistically uncertain, but it is a signal to take seriously.
Safety is a separate issue. Green tea as a beverage is generally safe in normal amounts. Concentrated supplements are not necessarily so: there are reports of gastrointestinal complaints, elevated liver enzyme levels and, in exceptional cases, serious liver damage at high doses. If you are considering taking a supplement, discuss it with your doctor first, especially if you are already taking medication.
Almost all research has been conducted in people in Asian countries who drink large amounts of green tea every day. Whether a few cups a week within a Western dietary pattern has any effect at all cannot be determined from the available research.
Based on a large Cochrane review (PMID 32118296, 142 studies, >1.1 million participants), supplementary epidemiological overviews (PMID 23840110, PMID 16582024) and mechanistic laboratory/animal studies (PMID 36142616, 30585192, 27634207). The Cochrane review is the primary source for human outcomes.