Tibolone, a synthetic drug used in many countries mainly to treat menopausal symptoms, reduces the risk of fractures and breast cancer, but also increases the risk of stroke in older women. Those are the findings of a study published in the August 14th issue of the New England Journal of Medicine.

Researchers recruited 4,538 women, between the ages of 60 and 85 years, in 22 countries for this randomized, double-blind study. Half were given 1.25 mg of tibolone daily, half were given an identical placebo. Those given tibolone had a decreased risk of vertebral fractures by 45% and the risk of other types of fractures by 26%. In addition tibolone reduced the risk of breast cancer. The effect of tibolone was comparable to those seen in women given the drugs tamoxifen or raloxifene.

"The reduction in breast cancer risk was a surprise to us," says Steven R. Cummings, M.D., of the California Pacific Medical Center Research Institute and the lead author of the study. "We don't know how tibolone reduces breast cancer risk. Understanding its mechanism might point us toward new ways to prevent the disease." In addition, the rate of colon cancer was lower in women who received tibolone. The mechanism for this effect is also unknown.

However, the drug came with a significant side effect: the rate of stroke was 2.3 per 1,000 woman-years higher in women taking tibolone than those taking placebo, representing a doubling of the risk of stroke. The study did not include women under age 60, so cannot determine whether tibolone significantly increases that risk in that age group.

Even so, Dr. Cummings says tibolone may still have some value for younger women. "Stroke is very rare among women under age 60, so although taking tibolone does slightly increase the risk for women in their 50's, the benefits of treatment for bothersome menopausal symptoms would outweigh those risks."

Dr. Cummings says the drug should not be given to women who have risk factors for stroke, such as hypertension or diabetes, nor should it be continued after age 65.

Tibolone is currently approved in 90 countries worldwide for the treatment of menopausal symptoms, and in 45 countries to prevent osteoporosis. It is not available in the United States.