A meta-analysis of studies into the relationship of tea drinking to the risk of developing breast cancer, has found a 20% reduction with high green tea consumption, with no comparable benefit for black tea (with some studies showing a small increased risk).

The anti-cancer benefits of tea are linked to the presence of water-extractable polyphenols which are much higher in green tea (30-40%) than black tea (3-10%). (Sun CL etal. Green tea, black tea and breast cancer risk: a meta-analysis of epidemiological studies. Carcinogenesis, doi:10.1093/carcin/bgi276).