What Is Tamoxifen and How Does It Work? Tamoxifen is used to treat breast cancer that has spread to other parts of the body (metastatic breast cancer), to treat breast cancer in certain patients after surgery and radiation therapy, and to reduce the chances of breast cancer in high-risk patients. Tamoxifen is a prescription medicine that is used to treat breast cancer in men and women. It is in a class known as a selective estrogen receptor modulator (SERM) and is used to treat hormone receptor-positive breast cancer. TAMOXIFEN (ta MOX i fen) prevents and treats breast cancer. It works by blocking the hormone estrogen in breast tissue, which prevents breast cancer cells from spreading or growing. Overview: Tamoxifen is commonly used to treat certain types of breast cancer. It is also used to lower your risk of invasive breast cancer and keep some breast cancers from coming back. It blocks. Tamoxifen blocks the actions of estrogen in breast tissue and is used to treat estrogen receptor-positive breast cancer in adults and to lower the risk of breast cancer in women with DCIS or at high risk. What Is Tamoxifen? Tamoxifen is an oral prescription medicine used to treat and help prevent hormone receptor–positive breast cancer. It is classified as a SERM, meaning it binds to estrogen receptors and can either block or mimic estrogen's action depending on the tissue. Tamoxifen is a hormone therapy drug. You might have it as a treatment for breast cancer. Find out about how you have it and possible side effects. Tamoxifen (brand names: Nolvadex, Soltamox) is a medicine that blocks the effects of the hormone estrogen (called a selective estrogen receptor modulator, or SERM). Tamoxifen is a first choice among doctors for treating women who have not yet gone through menopause. Tamoxifen is a nonsteroidal SERM of the triphenylethylene family and was structurally derived from diethylstilbestrol -like estrogens and antiestrogens such as chlorotrianisene and ethamoxytriphetol. [103][104][105][106] Initially, clomifene was synthesized, and tamoxifen was developed subsequently. [103][105][106] Tamoxifen is closely related. Tamoxifen is an antiestrogen drug used in the treatment of breast cancer. It blocks the activity of estrogen, helping to stop the growth of some tumors.