Tamoxifen is a medication primarily used to treat and prevent certain types of breast cancer, particularly in women with hormone receptor-positive tumors. It works by blocking the effects of estrogen in the breast tissue, which can help slow or stop the growth of cancer cells that rely on this hormone. In addition to its use in breast cancer treatment, tamoxifen is sometimes prescribed to reduce the risk of developing breast cancer in women at high risk. It is typically taken as a daily pill and may have side effects, including hot flashes and an increased risk of blood clots.