A synthetic nonsteroidal estrogen antagonist used in the prevention and treatment of breast cancer.Tamoxifen belongs to a class of synthetic agents known as selective estrogen receptor modulators (SERMs). By competing with naturally occurring estrogen for binding sites on tissue cells, tamoxifen inhibits the stimulant effect of estrogen on breast cancers. Tumors that have been shown by biochemical assay to be rich in estrogen receptors are most likely to respond to treatment. Since 1985, tamoxifen has been used to delay or prevent relapse in patients who have undergone surgery or irradiation for breast cancer. The drug has been found effective in reducing the risk of cancer recurrence or disease progression in women with or without axillary node metastasis. In women with extensive disease, tamoxifen therapy has been as effective as oophorectomy in retarding progression. Clinical trials of tamoxifen as a preventive agent in women at high risk of breast cancer have yielded conflicting results; current opinion is against using tamoxifen in women at low or normal risk of breast cancer because of side effects. Genomic analysis of healthy women who developed breast cancer while taking tamoxifen showed that tamoxifen reduced breast cancer incidence in women with the BRCA2 mutation, but not in those with the BRCA1 mutation. Women taking tamoxifen are at increased risk of endometrial carcinoma, stroke, deep venous thrombosis, pulmonary embolism, and cataracts. The danger of these adverse consequences is greatest in women older than 50. Long-term use of the drug is associated with recurrent vaginal candidosis. It is contraindicated during pregnancy because of the risk of fetal harm.