Women who take commonly prescribed drugs for osteoporosis such as Fosamax, Actonel or Didrocal (known as bisphosphonates) for five years or more lower their risk for hip fracture, but they may be at higher risk of certain kinds of fractures of their thigh bone.

A new study by researchers at St. Michael's Hospital and the Institute for Clinical Evaluative Sciences, published today in the Journal of the American Medical Association (JAMA) found, however, women at high risk for hip and other osteoporosis-related fractures should not stop taking bisphosphonates since the risk for thigh bone fracture caused by drugs is low.

Typical hip fractures caused by osteoporosis occur in the upper part of the femur close to the hip joint and are an important cause of disability, need for long-term care and even death in the elderly.

The risk of these kinds of fractures is reduced by bisphosphonate treatment, which was confirmed by this study.

Femur at risk

But the study also found that less common fractures lower down from the hip and closer to the middle of the femur -- sometimes called "atypical" or "unusual" fractures -- occurred more than 2.5 times as often in women who had taken bisphosphonates for more than five years than short-term users of the drug.

Study lead author, Laura Park-Wyllie, a pharmaceutical safety and outcomes researcher at St. Michael's, said prolonged use of bisphosphonates is associated with rare and unusual fractures of the femur but the proven benefits of the drugs for the much more common fractures due to osteoporosis usually outweigh the harm, especially in the initial years of treatment for osteoporosis.

“Women with osteoporosis, at high risk for osteoporotic fractures, should not stop taking bisphosphonate therapy because of the small risk of these thigh fractures," said Park-Wyllie.

She said the study was prompted by an increasing number of reports of thigh bone fractures among older women who have taken bisphosphonates for five years or more and by conflicting findings from small, observational studies.

Risk not significant

The St. Michael's-ICES study is the largest assessment of the issue to date. The researchers identified 205,466 women over age 68 who had been prescribed bisphosphonates between 2002 and 2008.

Of those, 716 women (0.35 per cent) had a fracture of the femur. These women were compared to other women of similar ages who had also been prescribed the drugs but who did not have femur fractures.

"Our study estimated that the risk of fractures to the femur was 0.13 per cent for women entering their sixth year on the drug -- or just over one in 1,000," Park-Wyllie said. "Use of bisphosphonates for less than five years was not associated with a significant risk of thigh fractures."

About 50 percent of women over 50 will suffer an osteoporosis-related fracture. The most common involve the wrist and the spine, but hip fractures can have some of the most severe consequences, with one in five of those women expected to die within 12 months of injury.