Women who take estrogen after menopause run a slightly higher risk of breast cancer, a 10-year study concluded. But researchers said women should not automatically abandon the hormone supplements.

An estimated 20 million prescriptions for estrogen are written each year for post-menopausal women to relieve the symptoms of menopause, fight the debilitating bone disease osteoporosis and battle heart disease.The study of more than 118,000 female nurses over a 10-year period indicated that women who take estrogen are 35 percent to 40 percent more likely to develop breast cancer than those who didn't take the hormone.

Only a small fraction of the women in the study developed breast cancer, so the increase in risk was considered small.

The added risk is only about half what a woman might face if her mother had breast cancer, said epidemiologist Dr. Graham Colditz of Boston's Brigham and Women's Hospital, lead author of the study in Wednesday's Journal of the American Medical Association.

Once women stop taking estrogen, the risk is eliminated, he said.

"I would not recommend that a woman change her current use of estrogen therapy based on these data alone," said co-author Dr. Meir Stampfer of the Harvard School of Public Health in Boston.

Women who are at high risk for breast cancer and no risk for heart disease might choose to avoid estrogen use, while women who run a high risk of heart disease and little risk of breast cancer might still use it, Colditz said.

The article analyzed data collected from 1976 to 1986. In all, 722 incidents of breast cancer were identified among the women.