Tamoxifen, a selective estrogen-receptor modulator, is an oral medication exhibiting both estrogen agonist and antagonist effects. Tamoxifen can be used to treat advanced breast cancer, to decrease the risk of recurrence of early-stage breast cancer, and for breast cancer prevention. Patients with an early-stage breast cancer with expression of the estrogen or progesterone receptors, or for whom no receptor result is known, benefit from five years of tamoxifen treatment. This treatment has been shown to decrease the annual risk of recurrence of breast cancer, and to decrease the risk of contralateral breast cancer by 47 percent. Tamoxifen has been studied as breast cancer prevention in patients at elevated risk of developing breast cancer (five-year risk of 1.66% or higher), decreasing the chance of developing an invasive or noninvasive breast cancer by approximately 50 percent. Newer antiestrogens, such as raloxifene, may have fewer side effects and similar effectiveness. Studies are underway to determine their effectiveness.
Osborne, C. K. (1998). "Tamoxifen in the Treatment of Breast Cancer." New England Journal of Medicine 339:1609–1618.