WASHINGTON (Reuters) - U.S. regulators said on Monday they were reviewing whether certain osteoporosis drugs, including those made by Merck Inc and Novartis AG, may be linked to a dangerously fast heartbeat.

The U.S. Food and Drug Administration said it would study the class of osteoporosis drugs known as bisphosphonates after two reports in the New England Journal of Medicine finding increased rates of serious atrial fibrillation (AF), a type of abnormal heartbeat, in patients who took either Merck's Fosamax or Novartis' Reclast.

"Upon initial review, it is unclear how these data on serious atrial fibrillation should be interpreted. Therefore, FDA does not believe that healthcare providers or patients should change either their prescribing practices or their use of bisphosphonates at this time," the FDA said in a statement.

Osteoporosis weakens bones and increases the risk of fractures.

Atrial fibrillation puts patients at risk for blood clots, which can cause strokes. AF is common in people over age 65, the population studied in the New England Journal article, the FDA said.

The article described increased rates of serious AF that were life-threatening, resulting in hospitalization or disability, the FDA said.

The FDA alert is part of a broader effort to notify the public earlier in the process when the agency reviews potential safety problems in drugs. It follows intense criticism of the FDA after Merck's painkiller Vioxx was pulled from the market in 2004 for links to heart attacks.

The FDA said it was seeking additional data for an in-depth review of the issue with the entire class of drugs. The review may take up to 12 months, the agency said.

A spokesman for Merck referred to a statement issued by the company when the New England Journal of Medicine study appeared in May. The drugmaker noted then that the link between Fosamax and serious AF was not statistically different than with a placebo, although it was numerically higher.

A Novartis spokesperson was not available for comment.

Bisphosphonates are used primarily to increase bone mass and reduce the risk of fractures in patients with osteoporosis. They also are used to slow bone turnover in patients with a disorder called Paget's disease and to treat bone metastases and lower blood calcium in cancer patients.

The other U.S.-approved bisphosphonates are Roche Holding AG and GlaxoSmithKline PLC's Boniva; Actonel, sold by Procter & Gamble and Sanofi-Aventis; Novartis AG's Zometa and Aredia; P&G's Didronel, and Sanofi's Skelid.

P&G spokeswoman Paula Koenigs said the company has reviewed its data and that "the data for Actonel don't suggest a safety concern."

Representatives of other companies that make bisphosphonates were not available for comment.