NEW YORK -- Patients whose bones are literally crumbling from osteoporosis are turning to the first drug that has the potential of reversing a disorder that thins bone density and affects 10 million Americans.

Doctors are hailing Eli Lilly and Co.'s Forteo as a breakthrough for patients whose bones are susceptible to fracture, sometimes from a stress as gentle as coughing, despite treatment with standard medicines.

The once-daily injectable medicine received regulatory approval in November for men and post-menopausal women at high risk for bone fractures.

Sales hit $29 million in the first half of this year, but could eventually reach as much as $800 million annually, according to Mehta Partners analyst Shaojing Tong.

"Forteo is a good drug that is addressing a great need," he said, noting that most people who suffer osteoporosis-related fractures fail to seek treatment for the disease.

Forteo is the first medicine to stimulate new bone formation. Most other drugs, including the current best-selling treatment, Merck & Co.'s Fosamax, work by stopping or slowing bone loss.

"Forteo has the potential to cure osteoporosis in many patients by rebuilding bone mass to the normal range and thereby ceasing new fractures," said Dr. Stephen Hodgson, a professor of medicine at the Mayo Clinic. Hodgson said he has no financial ties to Lilly.

Doctors are endorsing the drug despite a warning on the label, which says rats developed bone cancer after taking high doses for most of their lives. The medicine has not been linked to tumors in humans.

Lilly is counting on Forteo to help revive its earnings following the Indianapolis company's loss of U.S. patent protection on its flagship product, antidepressant Prozac.

But Forteo faces the prospect of competition from Preos, an osteoporosis drug that NPS Pharmaceuticals Inc. is developing. The company is hoping to seek U.S. approval for the medicine next year.

If Preos does not carry a rat-tumor warning, its sales could outstrip those of Forteo, analysts said.