University of Pennsylvania Cancer Center
Last Modified: May 18, 1998
Tamoxifen has been used for the past twenty years to treat breast cancer and prevent its recurrence. Knowledge of its benefit in preventing breast cancer remained unknown, however. Today researchers from the National Surgical Breast and Bowel Project (NSABP) presented the results of a trial examining the breast cancer prevention benefits of tamoxifen.
13,388 women participated in the placebo controlled, double-blinded randomized clinical trial. Through almost four years of follow-up, the study demonstrated that the drug reduces the incidence of breast cancer by 45% among women at high risk of the disease. "High risk" was determined by looking at each woman's family history of breast cancer, age, pregnancy history, and age at time of menstruation, among other factors.
Of those participating in the trial, 85 women on tamoxifen developed breast cancer, compared to 154 cases in the women assigned to placebo. The study did find an elevated risk of developing endometrial cancer (33 cases on tamoxifen, compared to 14 in placebo); pulmonary embolism (17 versus 6) ; and thrombosis (30 versus 19). Women under 50, however, experience no excess risk of side effects.