A drug used to treat certain types of breast cancer in women and men. It is also used to prevent breast cancer in women who have had ductal carcinoma in situ (abnormal cells in the ducts of the breast) and in women who are at a high risk of developing breast cancer. Tamoxifen is also being studied in the treatment of other types of cancer. It blocks the effects of the hormone estrogen in breast tissue, which may help keep breast cancer cells from growing. Tamoxifen is a type of selective estrogen receptor modulator (SERM). Also called tamoxifen citrate.