Park-Wyllie said the study was prompted by an increasing number of reports of thigh bone fractures among older women who have taken the drugs sold as Fosamax, Actonel or Didrocal for five years or more and by conflicting findings from small, observational studies.

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 per cent 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.

Source: St. Michael's Hospital