BOSTON — A medicine that strengthens elderly women’s fragile bones can cut their risk of spinal fractures in half, significantly reducing the painful and crippling effects of osteoporosis, a study concludes.

The medicine, called etidronate, reverses much of the slow loss of bone that underlies osteoporosis, the brittle-bones disease that afflicts an estimated 15 million to 20 million Americans.

The research shows that etidronate can prevent crushed vertebrae, which are extremely common in old age, especially among women. This fracture is the most frequent consequence of osteoporosis. However, there is no evidence yet that the treatment will stop broken hips, which are a less common but more serious hazard of the disease.

“This is really exciting,” said Dr. Nelson B. Watts of Emory University, the principal author of the study. “We can wait until somebody is already in bad shape and make them better. They won’t stand straight or play basketball. But the deterioration will be halted.”

Experts believe etidronate will soon join two other prescription drugs — estrogen and calcitonin — for routine treatment of the disease. Because of its apparent advantages, it could become the preferred medicine for many elderly victims.

“We think it’s extremely encouraging news,” said Sandra Raymond, executive director of the National Osteoporosis Foundation. “Our arsenal is very small. Hopefully now, based on these studies, we may have another option.”

The latest study, conducted at seven U.S. medical centers, was published in Thursday’s New England Journal of Medicine. In May, the journal carried a smaller study by European researchers that reached similar conclusions. Both studies were financed by Procter & Gamble’s Norwich Eaton Pharmaceuticals, which makes etidronate.

In the latest work, 429 women with osteoporosis were randomly assigned to take etidronate or placebos for two years. When it was over, the spinal bone density of those getting etidronate had increased 5 percent, and they suffered half as many vertebral fractures as the other women.