Selective estrogen receptor modulators (SERMs) and selective estrogen receptor downregulators (SERDs) are used in antiestrogen therapy for breast cancer. They block estrogen, which some breast cancers need to grow. SERMs can treat and sometimes prevent breast cancer, and also help in preventing osteoporosis. SERMs block estrogen in breast cells but can activate it in other cells, like bones and the uterus. Tamoxifen, Toremifene, and Raloxifene are examples of SERMs. SERDs, like Elacestrant and Fulvestrant, also block estrogen but additionally reduce the number of estrogen receptors. These drugs are used for late-stage hormone receptor-positive, HER2-negative breast cancers. SERMs and SERDs have different uses, including before surgery to shrink tumors, after surgery to prevent recurrence, and in late-stage cancer to slow its spread. They also come with side effects, similar to menopause symptoms, and can have serious risks.