Genistein is one of several known isoflavones. Isoflavones, such as genistein and daidzein, are found in a number of plants, with soybeans and soy products like tofu and textured vegetable protein being the primary food source. Soy isoflavones are a group of compounds found in and isolated from the soybean. Besides functioning as antioxidants, many isoflavones have been shown to interact with animal and human estrogen receptors, causing effects in the body similar to those caused by the hormone estrogen. Soy isoflavones also produce non-hormonal effects.

Isoflavones act as antioxidants to counteract damaging effects of free radicals in tissues. Isoflavones can act like estrogen in stimulating development and maintenance of female characteristics or they can block cells from using other forms of estrogen. Isoflavones also have been found to have antiangiogenic effects (blocking formation of new blood vessels), and may block the uncontrolled cell growth associated with cancer, most likely by inhibiting the activity of substances in the body that regulate cell division and cell survival (growth factors).

Studies show that groups of people who eat large amounts of soy-based products have lower incidences of breast, colon, endometrial, and prostate cancers than the general (US) population. Initial studies of soy isoflavone mixtures containing genistein, daidzein, and glycitein have found them safe for human use. Laboratory studies using animals models have shown that both soy and isoflavones can be protective against cancer when given during early life but can stimulate response to cancer-causing chemicals when given during fetal development or when circulating levels of estrogen are low (menopause).