Millions of women take alendronate, a drug that is designed to strengthen bones, fight osteoporosis and prevent bone fractures. But doctors didn't know how many years women could safely take the drug or how effective it was in long-term use.

But a new report in the Journal of the American Medical Association said that it may even keep working after women stop taking it.

One woman who took it was 58-year-old Judy Sheridan. She started four years ago when tests showed hints of osteoporosis.

"It is helping me because I've had a follow-up test a couple years after I started and my bone strength had improved," she said.

She even fell down a flight of stairs and only suffered brusies. And she can safely stay on the drug, Dr. Dennis Black of the University of California, San Francisco, said.

"The drug was safe to use for as long as 10 years. We didn't see any negative effects of long-term use," he said.

His study of about 1,000 women on the drug found some other surprises.

"On average, the women in the study had five years of previous alendronate use. They were then randomly assigned to get five more years of alendronate, or five years of placebo, a sugar pill."

After the second five years, tests showed that women on a placebo lost a little more bone, but fracture rates were essentially the same, "suggesting that for many women, they may be able to discontinue alendronate after five years and still maintain a five-year additional fracture benefit," Black said.

Women who already had fractures were at a high risk for more and should stay on the drug, he said.

The pharmaceutical company Merck provided funding for the study.