People who took a popular class of osteoporosis drugs known as bisphosphonates lived an average of five years longer than those who didn’t take them, according to a new study published online in 2011 in the Journal of Clinical Endocrinology & Metabolism.

“It certainly was an unexpected finding,” says study author Jackie Center, PhD, an associate professor at the Garvan Institute of Medical Research in Sydney, Australia. “We spent a year and a half trying to find a way to explain why this might not be a real finding, but there was nothing we could find that would explain it beyond the treatment.”

The Australian researchers analyzed data from 2,000 people in an ongoing osteoporosis study and found 121 women and men who had been taking bisphosphonates like alendronate, or Fosamax, and risedronate, or Actonel, for an average of three years.

Among that group, the death rate among women who were older than 75 when they took bisphosphonates was 69 percent lower than expected.

“We found it really surprising,” Dr. Center says.

Researchers say mortality rates are best determined based on 100 person-years. One death per 100 person years means for every 100 people, there would be one death per year. Using that as a measurement, researchers say women on bisphosphonates had a death rate of 0.76 percent per 100 person-years, which was lower than women taking hormone therapy (1.2 percent), women taking calcium and vitamin D (3.18 percent) and women who weren’t taking anything at all (3.46 percent).

Results were similar in men, but the number of men studied was smaller, so researchers say they can’t be as confident about that finding.

John Eisman, PhD, was another author of the study. He says initially his team thought this reduction in mortality risk could have been due to the fact that people seeking treatment were healthier and taking better care of themselves. “But everything we saw made us think these would be people who would do worse and in fact be more likely to die,” he explains.