A relatively inexpensive, easily obtainable drug may be effective in reversing osteoporosis, according to one of the largest studies ever of women with the common and frequently debilitating bone disorder.

The study reported in today's New England Journal of Medicine followed 423 women for two years. It found that etidronate, a drug now used to treat a less common skeletal disorder called Paget's disease, cut in half the women's rate of vertebral fractures, the most common type of fracture suffered by people with osteoporosis. The women's spinal bone density -- a measure of bone strength -- also increased.

"What we have is a new option for treating osteoporosis," said Charles Chesnut, a professor of medicine and radiology at the University of Washington Medical Center in Seattle, one of the study's authors. "It's safe, it's taken orally, it appears to be efficacious."

Another drug found to reverse osteoporosis, a hormone called calcitonin, is available only in injectible form in this country. It is also expensive, costing between $2,000 and $3,000 per year when used on a short-term basis, and studies indicate it replaces bone only during the first year of treatment. A cheaper, inhaled form of calcitonin is being studied.

Therapy with another hormone, estrogen, costs about the same astherapy with etidronate -- about $200 to $300 a year, according to Nelson Watts, an associate professor of medicine at Emory University in Atlanta and one of the study's authors. But estrogen works by preventing bone loss, rather than replacing bone in women who already have osteoporosis. It can also have side effects such as vaginal bleeding, and should not be used by women with breast cancer.

One-third to one-half of American women over age 50 suffer from osteoporosis, according to the National Osteoporosis Foundation. The group estimates that the disease, which breaks down bone tissue and is frequently painful, causes 1.3 million fractures each year.

The women in the study were randomly divided into four groups. All were given either phosphate, a chemical that increases the activity of cells that break down bone, or a placebo for three days. They were then given either etidronate or a placebo for the following two weeks.

The treatment was repeated once every three months. All the women took calcium when they were not taking the other drugs.

The researchers found half the rate of new vertebral fractures in women on etridonate compared to that of the other women. They also found the target women's spinal bone density increased between 4 and 5 percent, while that of the other women increased an average of only 1 percent -- an insignificant amount, Watts said.

Some women experienced nausea and diarrhea, but no other side effects were observed.

A study published in May in the New England Journal of Medicine also found etridonate effective in treating osteoporosis, but it only involved 66 women.

A study published two years ago in the Journal of Clinical Endocrinology and Metabolism found etidronate no more effective than estrogen or calcium supplements in decreasing the rate of vertebral fractures. But Chesnut said the study may have been flawed because the women were taking calcium along with the drug. Calcium prevents absorption of etidronate, which must be taken on an empty stomach.

Etidronate, also called didronel, is approved by the FDA for use in Paget's disease but not for osteoporosis. Officials at Norwich Eaton Pharmaceuticals, the drug's manufacturer, plan to apply for FDA approval for osteoporosis therapy soon.

Both the study's authors and other osteoporosis experts cautioned against indiscriminate use of etidronate for osteoporosis before it is approved by the FDA. At high doses, the drug can reduce deposition of calcium and other minerals in bone, weakening the skeleton.

"Based on current information, this is clearly the most promising drug for osteoporosis," Watts said. "But intermittent cyclical administration is important."

Louis Avioli, director of the division of bone and mineral diseases at Washington University in St. Louis, warned that many of the people who might be interested in taking etidronate -- older women whose diets might be deficient in Vitamin D -- are also at a higher risk of developing osteomalacia, a bone disease that high doses of the drug may cause.