Tamoxifen belongs to the class of anti-estrogens. The medicine is prescribed for breast cancer treatment. This type of medication is acting like an antagonist of estrogen receptor and completely prevents estrogen influence on tumors caused by this hormone. Blockading estrogen Nolvadex stops tumor growth. Tamoxifen treats cancer that has spread to other parts of the body. It can be used by women who are at high risk for breast cancer and by women with DCIS (after surgery and radiation) to decrease the risk of developing breast cancer.