Effects of soy phytoestrogens genistein and daidzein on breast cancer growth.

To determine whether genistein and daidzein, the major phytoestrogens in soy, can stimulate breast cancer growth. Systematic search through primary English-language literature on MEDLINE (1966-January 2001), EMBASE (1982-January 2001) and Current Contents (1998-January 2001). Genistein and daidzein at low concentrations were found to stimulate breast tumor growth in in vitro and in vivo animal studies, and antagonize the antitumor effect of tamoxifen in vitro. At high concentrations, genistein inhibited tumor growth and enhanced the effect of tamoxifen in vitro. Genistein and daidzein may stimulate existing breast tumor growth and antagonize the effects of tamoxifen. Women with current or past breast cancer should be aware of the risks of potential tumor growth when taking soy products.