If you’re one of the 40 percent of people older than 55 who take an antidepressant, you may want to start thinking about your bones, even if you don’t have a family history or other risk factors for osteoporosis.

The Problem

Several large, well-designed studies have shown that men and women who take antidepressants have an increased risk of fracture compared with people who don’t take these kinds of medications. The association seems to be particularly strong in a class of antidepressants known as selective serotonin reuptake inhibitors, or SSRIs. This class includes drugs like paroxetine (Paxil) and fluoxetine (Prozac). The jury is still out on whether other kinds of antidepressants carry the same risks.

The Studies

The newest study to sound the alarm was published in the May 2008 issue of the Journal of General Internal Medicine and it included nearly 93,000 women enrolled in the Women’s Health Initiative observational study. It showed that women on antidepressants had a 30 percent increased risk of sustaining a spine fracture and a 20 percent increased risk of sustaining fractures at other sites such as the pelvis, femur or hand, but no increased risk for hip or wrist fractures.

Two more studies, both published in the June 25, 2007 issue of the Archives of Internal Medicine, came to similar conclusions. In the first study, researchers from the University of Minnesota at Minneapolis found that women who took an SSRI daily had twice the risk of sustaining bone fractures as those who did not take SSRIs. In the second study, researchers at Oregon Health and Sciences University in Portland found that men who used SSRIs, but not other kinds of antidepressants, had lower bone mineral density.

The Riddle

Researchers don’t fully understand how antidepressants affect bones.

Stephen Honig, MD, the director of the osteoporosis center at the Hospital for Joint Diseases in New York City, says that bone cells have receptors for the neurotransmitter serotonin, and that SSRIs, drugs that increase the amount of serotonin in the spaces between nerve cells, may alter the balance between the constant buildup and breakdown of bone tissue in the body.

Kenneth Saag, MD, a rheumatologist at the University of Alabama at Birmingham who wrote a commentary on the two studies in the Archives, said that the link between SSRIs and fractures is serious and that patients taking SSRIs may need to be monitored with bone scans and may need to start treatment with drugs that treat osteoporosis called bisphosphonates.

It’s especially important to talk to your doctors about biphosphonate therapy to protect your bones if you have previously had fractures, are postmenopausal, are a heavy smoker, are at a high risk for falls, or if you have experienced significant weight loss or have a low body mass index. Calculate your BMI.