Tamoxifen for the prevention of breast cancer: current status of the National Surgical Adjuvant Breast and Bowel Project P-1 study.

Background: Initial findings from the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial (P-1) demonstrated that tamoxifen reduced the risk of estrogen receptor-positive tumors and osteoporotic fractures in women at increased risk for breast cancer. Side effects of varying clinical significance were observed. The trial was unblinded because of the positive results, and follow-up continued.