An advisory panel to the U.S. Food and Drug Administration wants time limits on the use of osteoporosis drugs, but could not agree on a figure.

The panel has voted 17 to 6 to recommend that the labeling for osteoporosis drugs known as bisphosphonates become more specific about how long they should be used.

The panel heard presentations from the manufacturers of the four currently approved bisphosphonates: alendronate (Fosamax, Merck), risedronate (Actonel, Warner Chilcott), zoledronic acid (Reclast, Novartis) and ibandronate (Boniva, Roche).

According to the FDA, 10 million people in the United States have osteoporosis, and another 34 million have osteopenia. Bisphosphonates are widely used, but there is growing uncertainty about how long they should be used, and even how effective they are in warding off fractures in people with osteoporosis.

The FDA requires all bisphosphonates used to prevent or treat osteoporosis to have warning labels stating that optimal duration of use hasn't been determined. Also all patients on bisphosphonate therapy should have their need for continued therapy re-evaluated periodically.

Bisphosphonates are prescribed to some 5 million patients annually to stave off or treat osteoporosis and are highly effective at reducing the risk of osteoporotic fractures.

Last year, the FDA required makers of bisphosphonate drugs to add a warning to their labels about a small increased risk of atypical femur fractures.

The panel was also concerned with the drug's link to deterioration of the jawbone. In 2005, the FDA added a warning on bisphosphonates about osteonecrosis of the jaw, a rare disease in which the bone in the jaw dies.

There is no clear answer on the long-term safety of the bisphosphonate therapy for the prevention and treatment of osteoporosis, the panel said. It has called for more studies to hone in on the long-term risks and benefits of the drug.

The FDA said an estimated 9 percent of patients take the drugs longer than three years, and fewer 1 percent take them longer than five years.

The first drug, Fosamax, was marketed by Merck in 1995. The worldwide sales last year were $7.6 billion.