Raloxifene reduces the risk of developing breast cancer for postmenopausal women with high estrogen levels, according to researchers at the University of California, San Francisco.

However, the osteoporosis drug -- which is sold as Evista and is not yet approved as a cancer drug -- produces no reduction in the incidence of breast cancer in women who have very low estrogen levels, the researchers said.

Raloxifene given to a selected group of women reduced their rate of breast cancer by 76 percent over a four year period, according to the study published in the Journal of the American Medical Association.

Previous research has shown that the risk of breast cancer rises with the increase of a woman's estrogen level. Raloxifene blocks the effect of estrogen on breast tissue.

In their study of 7,290 postmenopausal women, ages 80 or younger, suffering from osteoporosis and having no history of breast cancer, researchers gave the women either the recommended daily dose of 60 mg, a dose of 120 mg, or a placebo for 4 years.

Their blood was tested for estradiol levels, which is the most potent form of estrogen. Women who had estradiol levels in the upper third range of the group were found to be at high risk of developing breast cancer and benefited the most from raloxifene at any dosage level, according to the researchers.

Women with undetectable levels of estradiol had a similar risk of developing breast cancer whether they received raloxifene or a placebo, said the researchers.

Researchers concluded that 47 percent of the women in the study who developed breast cancer who had estradiol levels above 10 pmol/L would not have developed the disease if they had been taking raloxifene for 4 years.

"Measuring estradiol and treating those postmenopausal women who have high levels could substantially reduce their rate of breast cancer," said Dr. Steven R. Cummings, UCSF professor of medicine, epidemiology and biostatistics and Director, UCSF Clinical Research Program.

Other Sources: UCSF, JAMA