A class of widely prescribed drugs used to prevent and treat osteoporosis is indeed associated with an unusual type of thigh fracture, according to a large just-published European study, but researchers found the overall risk was so small that the benefit of the drugs outweighed the danger, The Wall Street Journal reported.

The question of whether the drugs, called bisphosphonates, are associated with atypical femur fractures has been closely studied the past few years, in the wake of reports from orthopedic surgeons that they were seeing an increase in cases.

These are severe fractures in which the thighbone snaps in two.

Experts are not sure why some patients who use the drugs get these fractures, or whether the drugs themselves, genetic factors or other variables are at work. Studies of the issue have resulted in conflicting reports. Last October, the federal. Food and Drug Administration (FDA) announced a change in labeling on the drugs to reflect the possible risk of atypical fractures in patients taking bisphosphonates.

In the current study, published in the New England Journal of Medicine, researchers analyzed data from all 1.5 million women in Sweden who were age 55 or older in 2008. They obtained X-rays of 1,234 of the 1,271 women with fractures and found 59 who suffered the more unusual kind of fracture.

The women with atypical fractures were also compared with 263 controls with fractures in a similar location. The researchers found that 78 percent of the women with atypical fractures took bisphosphonates, compared with 10 percent of controls.

Nonetheless, the researchers concluded that atypical fractures remain a rare event, and that the benefits of using the drugs outweighed the risks.

Bisphosphonates, which include brands such as Actonel, Zometa and Boniva, are used mainly by post-menopausal women as a way to prevent fractures that are associated with osteoporosis. Some 36.5 million prescriptions were dispensed for the drugs in 2010, according to health-care information firm IMS Health, and total US sales were more than $4.2 billion.

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