Weight-bearing exercise can help increase bone density to prevent osteoporosis. (Donna McWilliam/Associated Press)

Widely used osteoporosis drugs do not increase the risk of rare hip fractures, contrary to what previous case studies had suggested, a new study finds.

Bone-building drugs known as bisphosphonates have been taken by millions of postmenopausal women to treat osteoporosis.

In 2008, case reports started to surface about women taking the osteoporosis drugs to prevent broken bones who were showing up with "atypical" and serious fractures of their thighbones, which had snapped and sometimes splintered. In some cases, there was no trauma, such as a fall, before the bone broke.

"If we treated 1,000 osteoporotic women for three years, we estimate you would prevent 100 fractures," at a possible cost of one or fewer of the unusual bone breaks, said study author Dr. Dennis Black, a professor of epidemiology and biostatistics at the University of California, San Francisco.

The results were reported in Thursday's New England Journal of Medicine.

Fractures are important because of their association with an increased rate of death, illness, loss of mobility, independence and quality of life.

The drugs are widely considered safe, and the earlier case reports do not indicate a cause and effect, Black's study suggested. Sometimes the fractures occurred in people taking other medications as well as a bisphosphonate.

His work combined the results of three randomized trials involving 14,195 women taking the bisphosphonates drugs Fosamax or Reclast or a placebo for three to 10 years.

The researchers found 284 hip and leg fractures, including 12 unusual upper thigh fractures in 10 women.

"There are too few fractures for definitive proof. But what it does show is that these are very, very rare," Black said, adding it can't be ruled out that the medications might cause this type of injury.

U.S. research review to come

Few of the women took the drugs longer than three years, and many used a lower dose of Fosamax, Dr. Elizabeth Shane of Columbia University said in a journal editorial.

"The experience with bisphosphonates highlights the importance of postmarketing surveillance studies to detect the emergence of rare side effects of drugs that are widely used for very common chronic diseases," Shane wrote.

Shane suggested doctors should not rush to judgment and stop prescribing bisphosphonates.

However, they were urged to re-evaluate patients who are receiving long-term bisphosphonate therapy in the context of contemporary guidelines for starting treatment, progress while receiving therapy, current bone mineral density measurements, and risk factors for fracture.

No causal link has been established, agreed Dr. Kenneth Egol, a professor and vice chairman of orthopedic surgery at the New York University Hospital for Joint Diseases.

"We are going to look more closely at the biology of this," Egol told HealthDay News. "Is there anything that could potentially account for rare and unusual fractures? If we can find a reason why bones behave [in a way] that could explain fractures, that would go a long way to explain whether this is real or not."

Shane is co-chairing a task force by the American Society of Bone and Mineral Research that will analyze all published and unpublished data on the unusual fractures.

The study was funded by bisphosphonate makers Novartis and Merck. Several authors reported ties to various pharmaceutical companies.

With files from The Associated Press