(Adds Merck comment, paragraphs 8-9)

By Julie Steenhuysen

CHICAGO, April 28 (Reuters) - Two studies provide new evidence of risks associated with drugs commonly used to treat diabetes and osteoporosis, researchers said on Monday.

In one study, U.S. researchers found women who took Merck & Co Inc's <MRK.N> osteoporosis drug Fosamax, or alendronate sodium, were nearly twice as likely to have a type of abnormal heartbeat known as atrial fibrillation.

In another, Swiss researchers found the diabetes drugs Avandia, or rosiglitazone, from GlaxoSmithKline Plc <GSK.L> and Actos, or pioglitazone, from Takeda Pharmaceutical Co Ltd <4502.T> can more than double the risk of bone fractures.

The studies, published in the Archives of Internal Medicine, underscore the need for patients to weigh the risks and benefits of drugs carefully, said Dr. Jane Cauley of the University of Pittsburgh, who wrote a commentary.

For women with osteoporosis, the study by Dr. Susan Heckbert of the University of Washington and Group Health in Seattle adds to concerns that drugs in bisphosphonate class raise the risk of atrial fibrillation, which can lead to blood clots and strokes.

Heckbert's team compared 719 women who had atrial fibrillation to 966 women who did not and found using Fosamax was linked with an 86 percent higher risk of newly detected atrial fibrillation compared with never using the drug.

Last fall, the U.S. Food and Drug Administration said it would study drugs in this class after two reports in the New England Journal of Medicine found increased rates of serious atrial fibrillation in patients who took either Fosamax or a drug by Novartis <NOVN.VX> known as zoledronic acid or Reclast.

Merck said in a statement its own randomized clinical trial did not find a statistically significant difference in serious cases of atrial fibrillation in people to took Fosamax and those who did not.

"We strongly recommend that if patients have concerns about Fosamax that they talk to their physician," the company said.

BONE VS HEART

Bisphosphonates are used primarily to increase bone mass and cut the risk of fractures in patients with osteoporosis.

But some researchers think they may trigger a number of small changes that could affect the chambers of the heart known as atria, altering the heart beat. Heckbert's team said these possibilities need to be studied.

"The benefits of fracture prevention in patients at high risk for fracture will generally outweigh the possible risks of atrial fibrillation," Heckbert and colleagues wrote.

But for women with modest fracture risks and known heart risks "it is important to carefully weigh the benefits against the possible risk of atrial fibrillation," they wrote.

In the diabetes study by Christian Meier of University Hospital Basel, people who took Avandia or Actos had double or triple the odds of non-spine fractures.

The risk for fracture was increased among patients who took the drugs for about 12 to 18 months and the risk was highest for those with two or more years of therapy.

Cauley said that while the evidence of fracture risk is strong for the diabetes drugs, the link between bisphosphonates and atrial fibrillation is murkier.

She said a large Danish study published last month in the British Medical Journal found no association between bisphosphonates and atrial fibrillation.

"I think the overall evidence is that there is no association between bisphosphonates and atrial fibrillation," she said in a telephone interview.

"On the other hand, with the diabetes drugs, I think there is indeed a possible fracture risk association," added Cauley, who has received research support from Merck. (Editing by Maggie Fox and Eric Walsh)

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