A long-term international study shows that the drug raloxifene is almost as effective in preventing breast cancer among high-risk women as the related drug tamoxifen, but with far fewer dangerous side effects.

After tracking more than 19,000 women for nearly seven years, the Study of Tamoxifen and Raloxifene, known as STAR, determined that raloxifene is about 75 percent as effective as tamoxifen over the long run, but is much less likely to cause endometrial cancer, cataracts or blood clots.

Those side effects have kept many women from using tamoxifen to guard against breast cancer, even if they have a strong family history of it, said Dr. D. Lawrence Wickerham of Allegheny General Hospital, who presented the findings today at the annual meeting of the American Association for Cancer Research.

The relative effectiveness of raloxifene, combined with the fact that it has only half the rate of uterine cancer and helps to prevent osteoporosis, makes it a useful preventive drug, particularly for younger postmenopausal women who have an intact uterus, he said.

Over several years, the study estimated, tamoxifen reduces breast cancer in high-risk women by about 50 percent, while raloxifene cuts the risk by 38 percent.

The STAR effort is overseen by the National Surgical Adjuvant Breast and Bowel Project, which is headquartered in Pittsburgh, but also involves institutions and patients in Philadelphia, Georgia, North Carolina, Texas, California, Colorado, Illinois, Connecticut and Canada.

The study shows that both drugs are effective in preventing breast cancer in postmenopausal women with a family history of the disease, but that each may have advantages for certain subsets of women, Dr. Wickerham said.

"The bottom line is that postmenopausal women who have increased risk for breast cancer now have some options to substantially reduce that risk," he said. "It will allow us to take a major step forward in making breast cancer a preventable disease."