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Mon Oct 1, 2007 5:00pm EDT

WASHINGTON (Reuters) - The U.S. Food and Drug Administration said on Monday it was reviewing reports of abnormal heartbeats in patients who took medicines in a class of osteoporosis drugs called bisphosphonates.

"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," an FDA statement said.

A study and letter published in the New England Journal of Medicine in May described increased rates of serious atrial fibrillation, a type of abnormal heartbeat, in patients who took either of two bisphosphonates -- Merck & Co Inc's Fosamax or Novartis AG's Reclast.

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

Bisphosphonates are used primarily to increase bone mass and reduce fracture risk 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.