"For people who either don't want to take, or can't take, oral drugs, this is a really good alternative that's really effective," said Dr. Dennis Black of the University of California San Francisco, who led the study.

"For the first time, women could have the option of being treated once a year for osteoporosis, instead of having to remember to take a weekly pill."

Reclast, sold outside the United States as Aclasta, is already approved in more than 50 countries to treat the abnormal bone growth of Paget's disease. It is under review in the United States for osteoporosis.

Earlier Reclast studies in women with osteoporosis showed that one 15-minute infusion increased bone density for a year.

In the three-year study, also sponsored by the Swiss drugmaker, one infusion of the drug cut the risk of spine fractures by 70 percent and the chance of hip fractures by 41 percent. Conventional oral drugs typically produce a 40 to 50 percent reduction in spine bone breakage.

The results are "impressive," Dr. Juliet Compston of Britain's University of Cambridge wrote in a commentary in the New England Journal of Medicine, where the study appears.

However, serious atrial fibrillation — an abnormal heart rhythm that can increase the risk of stroke — was nearly three times more common among the 3,889 volunteers getting the Novartis drug than among the 3,876 given placebo infusions. One in 77 Reclast patients developed the problem.

"Everyone is trying to figure out if this is a true effect of the drug, or just a random event," said Black, who has received grants from Novartis.

Doctors are not sure if it is a side-effect because most cases of atrial fibrillation surfaced more than 30 days after treatment, Black.

A letter in the same issue of the Journal reports that a new analysis of a study of Merck's Fosamax, another bisphosphonate, found that 1 percent of the placebo patients experienced serious atrial fibrillation compared to 1.5 percent for the women getting the pill, known generically as alendronate.

"Thus, it is a potential concern, and a causal relationship must be given serious consideration," Compston said.

The new study found that the annual infusions of Reclast, known generically as zolendronic acid, produced other side-effects.

Overall, 31.6 percent of the Reclast recipients had fever, joint and muscle pain, headache, or flu-like symptoms after their injections, compared to 6.2 percent who got placebo injections.

But the chance of having any of those symptoms dropped to 6.6 percent with the second yearly injection, and 2.8 percent with the third.

Doctors would prefer a once-a-year treatment for osteoporosis because patients do not always take other medication regularly.

Bisphosphonate pills are supposed to be taken on an empty stomach with a full glass of water, and the patients must be upright for at least 30 minutes. As a result, most patients skip at least 20 percent of their pills.

Osteoporosis affects millions of women worldwide.