Tamoxifen is used for treating breast cancer that has spread to other sites in the body. It is also used along with other medicines to treat other types of breast cancer. It is used in women who are at high risk for breast cancer and in women with DCIS (after surgery and radiation) to decrease the risk of developing breast cancer. Tamoxifen is an antiestrogen. It works by blocking the effect of estrogen on certain tumors. This may prevent the growth of tumors that are activated by estrogen.