Women who have used Fosamax are nearly twice as likely to develop the most common kind of chronically irregular heartbeat (atrial fibrillation) than are those who have never used it, according to research from Group Health and the University of Washington published in the April 28 Archives of Internal Medicine.

Merck markets Fosamax, the most widely used drug treatment for the bone-thinning disease osteoporosis, explained study leader Susan Heckbert, MD, PhD, MPH, a professor of epidemiology and scientific investigator in the Cardiovascular Health Research Unit at the University of Washington. The Food and Drug Administration (FDA) approved the first generic versions (called alendronate) in February.

“We studied more than 700 female Group Health patients whose atrial fibrillation was first detected during a three-year period,” said Dr. Heckbert. She and her colleagues compared those women to over 900 randomly selected female Group Health members matched on age and high blood pressure to serve as controls.

“Having ever used alendronate was associated with an 86 percent higher risk of newly detected atrial fibrillation compared with never having used the drug,” said Dr. Heckbert, who is also an affiliate investigator at the Group Health Center for Health Studies.

Osteoporosis mostly affects older women and can set the stage for fractures that can impair the quality of their lives, said Dr. Heckbert. “Careful judgment is required to weigh the risks and benefits of any medication for any individual patient,” she added. “For most women at high risk of fracture, alendronate’s benefit of reducing fractures will outweigh the risk of atrial fibrillation.”

However, said Dr. Heckbert, “women who are at high risk of fractures but also have risk factors for atrial fibrillation — such as heart failure, diabetes, or coronary disease — might want to discuss alternatives to alendronate with their health care providers.” Other medications that can lower the risk of fractures include estrogen, she said. But the Women’s Health Initiative, on which she has also served as an investigator, showed other heart risks from hormone therapy combining estrogen with progesterone.

The National Heart, Lung, and Blood Institute funds Dr. Heckbert’s Atrial Fibrillation Study, which collects data on all Group Health patients as they are first diagnosed with atrial fibrillation. The study aims to find new factors that raise the risk of developing this quivering of the heart’s upper chambers (atria).

About one in 100 people — and nearly nine in 100 people over age 80 — have atrial fibrillation, said Dr. Heckbert. In many cases, atrial fibrillation has no symptoms, and it isn’t necessarily life threatening. But it can cause palpitations, fainting, fatigue, or congestive heart failure.

Atrial fibrillation can also make blood pool — and sometimes clot — in the atria, said Dr. Heckbert. When parts of clots break off and leave the atria, they can lead to embolic strokes, as happens in over 70,000 Americans a year. That’s why atrial fibrillation is often treated with the anticoagulant warfarin. Other results from her study have suggested that maintaining a healthy body weight may help protect people from atrial fibrillation.