Tamoxifen is an estrogen blocking agent that has recently been found to reduce the risk of breast cancer in women by as much as 40-50 percent. This remarkable finding may have implications for prostate cancer in men due to the effects of testosterone on the prostrate gland. The exact mechanism of action is unknown, but the drug serves to block estrogen receptors that allow cancer cell formation when bound by estrogen. Side effects include an increase in the incidence of uterine cancer, and tamoxifen has been reported to enhance the formation of cataracts.