Long-time users of osteoporosis drugs have a sharply higher risk of suffering thigh-bone fractures, a new Canadian study shows.
At the same time, the drugs, known as bisphosphonates, are effective at reducing the risk of much more common hip fractures, the researchers confirmed.
"If a woman has osteoporosis and is at high risk of fracture, she should not stop taking this treatment because the benefit outweighs the risk," Laura Park-Wyllie, a pharmaceutical safety and outcomes researcher at Li Ka Shing Knowledge Institute of St. Michael's Hospital in Toronto, said in an interview.
"But women who are not considered at high risk may want to reconsider," she said.
The research, published in Wednesday's edition of the Journal of the American Medical Association Journal, involved 205,466 women over the age of 68 who had been prescribed bisphosphonates in Ontario from 2002 to 2008.
Of that total, 716 women (0.35 per cent) who had been taking the drugs for five years or more suffered a fracture of the femur, or thigh bone. That type of break is considered atypical. The femur-fracture rate was 2.7 times higher in women taking bisphosphonates than in those of a similar age who were not taking the drugs.
Far more typical in sufferers of osteoporosis are broken hips, wrists and spinal fractures. About half of women over the age of 50 will suffer an osteoporosis-related fracture.
Dr. Park Wyllie said there has been considerable interest in reports of fractures of the thigh bone in users of bisphosphonates because, intuitively, breaking the femur, the largest bone in the body, would require considerable force.
The new findings - the first to show the association in a large population - suggest that bisphosphonates are making the thigh-bone quite brittle, but it is not clear why. "Unfortunately, we don't really have an understanding of how and why these atypical fractures are occurring," Dr. Park-Wyllie said.
Osteoporosis is a disease characterized by low bone mass and deterioration of bone tissue. It affects an estimated two million Canadians, including about one in four postmenopausal women and one in eight men over the age of 50.
In the new study, 9,723 (4.7 per cent) of long-time users of bisphosphonates suffered a hip fracture. That is 24 per cent lower than women of a similar age who were not prescribed the drugs.
"There is actually lot of evidence that these drugs are underused in those at high risk," Dr. Park-Wyllie said.
Risk factors for osteoporosis include age, being postmenopausal, family history, having a previous fracture and use of certain medications.
Bisphosphonates are a family of drugs used to prevent and treat osteoporosis. There are four currently approved for use in Canada: alendronate (brand name Fosamax), etidronate (Didrocal), risedronate (Actonel), taken in pill form, and zoledronic acid (Aclasta), an intravenous drug.
Bisphosphonates work by slowing the destructive action of osteoclasts (bone-eroding cells), allowing osteoblasts (bone-building cells) to work more effectively.