Two research reports suggest a possible link between two bone-building drugs and irregular heart rhythms in a small number of women who take the medicine.

The signs of a problem were more pronounced with Reclast, a drug made by Novartis AG and given through a once-a-year, 15-minute intravenous infusion. But there was a hint of similar trouble in a few women who took the leading osteoporosis pill, Fosamax by Merck & Co. The two drugs are in the same class.

The safety question caught researchers by surprise. While uncertain how big a worry it might be, they agreed the overall risk is small. Specialists said women at high risk for bone breaks _ the main target of these osteoporosis drugs _ should keep taking them as prescribed.

But several experts said they'd be cautious about those who also are at risk for a condition called atrial fibrillation, an irregular heart rhythm that can cause strokes.

The two separate reports published Thursday in The New England Journal of Medicine point to elevated rates of serious episodes of that heart condition in women who took Reclast and Fosamax.

"For the first time, there may be a side effect," said a researcher involved in both studies, Dr. Steven Cummings of California Pacific Medical Center Research Institute. Until now, people have assumed Fosamax "was completely safe and could be given to almost anybody."

Fosamax, the Merck brand name for alendronate, is now used by an estimated 1.8 million American women. In a letter to the medical journal, Cummings reported evidence of the heart problem found in a recent review of a 1997 Merck-sponsored study of postmenopausal women on Fosamax.

There appeared to be 50 percent more risk of the serious heart rhythm in women who took the daily pill than among those who didn't take it. About half of the 6,459 women took Fosamax, and 47 developed atrial fibrillation, compared to just 31 cases among the other women.

However, the finding, while not statistically definitive for Fosamax, worried some researchers because it is in line with the results of a new study published in the same issue of the medical journal.

This study of 7,736 postmenopausal women with bone-thinning osteoporosis focuses on Reclast. Novartis recently won approval to sell Reclast, known generically as zoledronic acid, for Paget's disease, another bone condition. The company hopes to get an OK later this year to sell it for osteoporosis use.

The new study, funded by Novartis, shows that Reclast works at least as well as existing drugs in the same class, researchers say. However, the risk of a serious case of irregular heart rhythm was more than double that in the other patients _ 50 cases in the drug-taking half, compared to 20 cases in the others.

Researchers cautioned about overestimating the importance of the heart rhythm problem. The Reclast study showed little apparent difference in overall cardiac deaths and the overall risk of the rhythm condition remained small.

Doctors made available by Merck and Novartis said the side effect could be a statistical fluke or just a product of aging. They said earlier studies showed no sign of the possible side effect.

For now, it's unknown if the possible risk applies to other drugs in the class known as biphosphonates. Future studies are expected to help clarify the risk.