The evidence is contradictory. Soybeans contain estrogen-like compounds called isoflavones that, in animal studies at least, fuel the growth of cancer cells and reverse the inhibitory effects that drugs such as tamoxifen (Nolvadex and generic) have on that growth. Yet Asian women, who tend to consume lots of soy foods, have a lower incidence of breast cancer than do women in Western countries. On balance, it's probably safe for most women to consume soy products as long as they don't go overboard (no more than about 100 mg of isoflavones a day). But women who already have breast cancer or are at high risk for it should limit their consumption.