Selective estrogen receptor modulators, or SERMs, are a class of compounds that can act as estrogen receptor (ER) agonists in some tissues while acting as ER antagonists in others. SERMs are being evaluated and used to treat and prevent such diseases as breast cancer, osteoporosis, and cardiovascular disease. Currently, three primary SERMs are used clinically, which include tamoxifen, toremifene (triphenylethylenes), and raloxifene (a benzothiophene). Tamoxifen and toremifene have beneficial effects on bone and serum lipids, and are currently used to treat breast cancer. Both have stimulatory effects on the uterus. Raloxifene, indicated for the treatment and prevention of osteoporosis, also has beneficial effects on bone and serum lipids, but does not stimulate the uterus. All three are associated with venous thromboembolism and hot flashes. New SERMs to treat and prevent breast cancer, osteoporosis, and cardiovascular disease are undergoing clinical development, including idoxifene, droloxifene, ospemifene, lasofoxifene, arzoxifene, and MDL 103,323.