Tamoxifen works against breast cancer by interfering with the activity of the hormone estrogen. Many breast cancers rely on the presence of estrogen to continue growing. Researchers have previously determined that women who take tamoxifen following a diagnosis of breast cancer, have a lower rate of breast cancer development in the opposite breast. This information led researchers to believe that tamoxifen may reduce the rate of breast cancer in women who were at an increased risk for developing the disease. The BCPT was designed to answer that question.
Many factors can contribute to the degree of risk for developing breast cancer.
BCPT participants who were placed in the tamoxifen group had more than twice the chance of developing endometrial cancer compared with women on placebo. This increased risk was equal to the risk that was expected for postmenopausal women who are taking single-agent estrogen replacement therapy. Endometrial cancer can be life threatening if not detected in the early stages, when treatment is very effective. Women who have undergone hysterectomies eliminated their risk for endometrial cancer. Women on tamoxifen were also more likely to develop blood clots in other major veins than women taking placebo.