Pfizer Inc.’s breast-cancer drug Aromasin worsened bone loss in post-menopausal women, raising the chance of fractures and calling into question whether the pill’s prevention benefits outweigh its risks.

In a trial among 351 women at risk of developing breast cancer, those who received Aromasin lost about three times more bone-mineral density after two years than those who took a placebo, researchers led by Angela Cheung at Toronto’s University Health Network wrote Tuesday in The Lancet Oncology.

The result contradicts earlier studies that suggested Aromasin, also known as exemestane, may help stimulate bone growth. It may also cause doctors and patients to question whether the cancer-preventing benefits associated with the drug outweigh the risks in healthy women, said Jane Cauley, a professor of epidemiology at the University of Pittsburgh.

“One might not be too reassured about the use of exemestane in the prevention setting,” Cauley wrote in an editorial accompanying the research.

Aromasin has been used since 1999 to treat breast cancer and stop it from returning. A study last year found it can also be used to prevent tumors in women at risk of the disease. While Aromasin isn’t approved for prevention of breast cancer, U.S. doctors are allowed to prescribe approved drugs for other uses.

The treatment generated $361 million in sales last year for New York-based Pfizer, the world’s biggest drugmaker. The company lost patent protection last year for the medicine in the United States, Europe, and Japan.

Not Approved Use

“The use of Aromasin as preventative therapy in post-menopausal women at high risk for developing breast cancer is not included in the Aromasin label in the U.S. and Europe, as well as other countries,” Jenifer Antonacci, a Pfizer spokeswoman, said in an e-mail. “It’s important to note that these data are in the National Comprehensive Cancer Network guidelines relating to the management of post-menopausal women at high risk for developing breast cancer.”

In the study announced Tuesday, researchers weren’t able to assess the long-term risk of fractures because the women were followed only for two years. Previous studies have shown that women treated for as much as five years with so-called aromatase inhibitors — the class of drugs including Aromasin — have no increase in their risk of fractures when compared with placebo or no treatment, said Paul Goss, a professor of medicine at Harvard Medical School who participated in the research.

“There are no clinically actionable data presented,” Goss said in an e-mail. “The bone quality findings are of interest to researchers but need to be interpreted with caution until data with longer treatment, and after treatment has stopped, have been demonstrated.”

The study was funded by the Canadian Breast Cancer Research Alliance.

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