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One of the largest breast cancer studies of its kind drew to a conclusion in spring 2006 and made its highly anticipated results known. The Study of Tamoxifen and Raloxifene, or STAR, under the auspices of the National Surgical Adjuvant Breast and Bowel Project supported by the National Cancer Institute, was designed to determine whether the osteoporosis prevention and treatment drug raloxifene (Evista®) is as effective as tamoxifen (Nolvadex®) in reducing breast-cancer risk. In 1998, tamoxifen was shown to reduce the chance of developing breast cancer by about half in a study of more than 13,000 premenopausal and postmenopausal women at increased risk of breast cancer. The U.S. Food and Drug Administration (FDA) approved the use of tamoxifen to reduce the incidence of breast cancer in women at increased risk of the disease. For its part, raloxifene was shown to reduce the incidence of breast cancer in a large osteoporosis trial.

STAR confirmed that raloxifene and tamoxifen are equally effective at preventing invasive breast cancer, reducing the risk by 50 percent in woman at high risk. In addition, women in the study who took raloxifene and were followed for an average of four years had 36 percent fewer uterine cancers and 29 percent fewer blood clots than women who took tamoxifen. In late 2007 the FDA approved the use of raloxifene to prevent breast cancer in postmenopausal women who have osteoporosis or are at high risk of invasive cancer.

Begun in 1999, STAR studied postmenopausal women who were at least age 35 and had an increased risk of breast cancer as determined by their age, family history of breast cancer, personal medical history, age at first menstrual period and age at first live birth. Each of the 19,000 participants was randomly assigned to receive either tamoxifen or raloxifene daily for five years and get regular follow-up examinations.