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Tamoxifen is a Selective Estrogen Receptor Modulator (SERM), one of a group of drugs that block the effect of estrogen on breast tissue. Tamoxifen is approved for use in both pre- and post-menopausal women. When taken for five years, it reduces breast cancer risk by up to 40%; this protective effect continues beyond the five-year treatment period. Based on this evidence, five years of tamoxifen is now recommended to reduce the risk of breast cancer in high-risk women.
Tamoxifen offers additional benefits beyond treatment. In studies of BRCA mutation carriers who were diagnosed with cancer in one breast and took tamoxifen, the risk for breast cancer in their healthy breast was reduced by up to 50%; this protective effect occurred even among BRCA1 mutation carriers who tended to develop ER-negative disease. Tamoxifen may also protect bone density and reduce osteoporosis risk in postmenopausal women who cannot take hormone replacement therapy. Side effects of tamoxifen include an increased risk of endometrial cancer and blood clots.
High-risk women who wish to consider tamoxifen to lower their risk for breast cancer should discuss the benefits, risks and limitations with their health care providers.