tamoxifen (təmŏk´sĬfĕn´), synthetic hormone used in the treatment of breast cancer. Introduced in 1978, tamoxifen is used to prevent recurrences of cancer in women who have already undergone surgery to remove their tumors and to control metastatic breast cancer. In breast tissue, tamoxifen blocks the effect of estrogen on cancerous cells that need estrogen to grow; not all breast cancers are affected by tamoxifen and other selective estrogen receptor modulators. In postmenopausal women with breast cancer, aromatase inhibitors, which block the production of estrogen outside the ovaries, are typically used instead of tamoxifen. In 1998, a large study of healthy women at high risk for breast cancer showed that tamoxifen can also prevent first occurrence of the disease; women who took the drug had a 45% lower incidence of cancer that those who received a placebo, and tamoxifen is now also used to reduce the risk of breast cancer in high risk patients. Adverse effects include and increased risk of uterine and endometrial cancer, blood clots, stroke, and cataracts.