Women following treatment with bisphosphonate alendronate (Fosamax) for their osteoporosis problems are twice as likely to develop a common form of irregular heartbeat or atrial fibrillation, compared to those who have never taken the drug, findings of a new study say.

Generic versions of alendronate were approved two months ago in February 2008, by the U.S. Food and Drug Administration for the treatment of osteoporosis.

“We’re not saying that this drug should be stopped and we certainly do not feel that patients should stop taking the drug. But we did detect this adverse effect,” study leader and professor of epidemiology and researcher at the Cardiovascular Health Research Unit at the University of Washington, Dr Susan Heckbert told WebMD.

Atrial fibrillation can lead to palpitations, fainting, fatigue or congestive heart failure. It can also lead to blood clots and stroke. The condition becomes more common with age and can be treated with medication. About 1 in 100 people, and nearly 9 in 100 over the age of 80, have atrial fibrillation.

Some 70,000 Americans a year have embolic strokes, which can be caused by atrial fibrillation where the blood "pools" and sometimes clots in the atria (chambers of the heart), from which a piece breaks off and causes the stroke.

For her study, Dr. Heckbert and her colleagues from Group Health analyzed 719 women with diagnosed atrial fibrillation that began taking the drug between 2001 and 2004, and 966 women who were the same age but did not have the condition. According to the findings, there was an 86 percent higher risk of newly found atrial fibrillation in those who had used Fosamax compared with those who had never used it.

“Having ever used alendronate was associated with an 86% higher risk of newly detected atrial fibrillation compared with never having used the drug. Careful judgment is required to weigh the risks and benefits of any medication for any individual patient. For most women at high risk of fracture, alendronate's benefit of reducing fractures will outweigh the risk of atrial fibrillation,” Dr. Heckbert said.

Dr. Heckbert’s research follows two previous studies, including one in the New England Journal of Medicine that showed that atrial fibrillation is an unexpected side effect of bisphosphonates.

Fosamax’s manufacturer, Merck & Co. responded to the findings with the same response given about the previous studies – there is not enough evidence to say Fosamax treatment is dangerous. A representative for the company also pointed to a study in last week’s British Medical Journal that showed no link between atrial fibrillation and bisphosphonates (a class of drugs used to treat osteoporosis that includes Fosamax).

The company also notes that randomized clinical trials are “the gold standard” for evaluating drug safety and efficacy, while Dr. Heckbert’s study was an observational analysis and its results could be limited.

The statement continues: “We strongly recommend that if patients have concerns about Fosamax that they talk to their physician. Osteoporosis is a serious medical condition and requires appropriate treatment with physician oversight. Their physician is in the best position to understand the needs of the patient and explain the benefits and risks of any given therapy to the patient.”

On the other hand, Dr. Christine Himes Fordyce a Group Health family practitioner said: “This study will help medical teams better inform their patients about the risks associated with Fosamax, helping us make the best treatment decisions for managing osteoporosis. Now with this increased understanding of potential irregular heartbeats, both physicians and their patients should be alert to any problems, report them immediately, and treat them appropriately.”

According to a Surgeon General’s report in 2004, about 44 million Americans suffer from osteoporosis, women more than men. After age 50, one in two women will break a bone as a result of osteoporosis. But the risks climb dramatically with age, particularly after 80.

Dr. Heckbert’s study appeared in the April 28 issue of the Archives of Internal Medicine.