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A drug that needs to be injected only once a year can sharply reduce fractures in women suffering from the bone-weakening disease osteoporosis, according to a new study.

Annual treatment with zoledronic acid produced impressive results in a large clinical trial, including:

A 70-per-cent reduction in spine fractures;

A 41-per-cent drop in broken hips;

A 25-per-cent reduction in other fractures, such as broken wrists.

On the downside, the drug appears to increase the risk of atrial fibrillation, a heart condition that greatly increases the risk of stroke.

"The data on fracture prevention is impressive and definitive," said Sophie Jamal, an endocrinologist at St. Michael's Hospital in Toronto who conducts research on osteoporosis but was not directly involved in the study. "But we need to find out more about heart irregularities."

The research, published in today's edition of the New England Journal of Medicine, was conducted by a team led by Dennis Black, of the department of epidemiology and biostatistics at the University of California, San Francisco.

The study involved 7,765 postmenopausal women aged 69 to 85, half of whom received zoledronic acid (brand name Aclasta), and the other half who received a placebo.

The women received a 15-minute infusion of the drug once a year over a three-year period. A single dose of the drug costs $645, according to the maker, Novartis Pharmaceuticals Canada Inc.

However, Aclasta has not yet been approved in Canada as a treatment for osteoporosis. (The drug is currently used to prevent the spread of cancer in the bones of breast-cancer sufferers and is given monthly.)

Aclasta belongs to a class of drugs known as bisphosphonates, which slow the breakdown of bones and the loss of bone density, which occur when people age.

Bisphosphonates in pill form, such as alendronate (brand name Fosamax) and risedronate (Actonel), are popular treatments for osteoporosis. (Supplements of calcium and vitamin D are also helpful.)

But Dr. Black said patients taking these other bisphosphonate pills, which must be taken once a week, have poor compliance, largely because the medications require a particular ritual, including taking the drugs on an empty stomach, drinking a full glass of water and then standing upright for at least 30 minutes afterward.

A single, yearly infusion of Aclasta would probably improve the efficacy of the drugs, he said. (While the new study doesn't compare the oral and infused forms head to head, the yearly drug reduced fractures at twice the rate as the weekly one did in similar studies.)

Dr. Jamal said some patients will be reluctant to take a needle and a long-lasting drug, but she predicted that it will be popular. "This is an amazing thing for patients because it offers another therapeutic option," she said.

Osteoporosis is a disease characterized by low bone mass and deterioration of bone tissue, which increases the risk of fracture. But it is largely a silent condition because people don't really suffer until they fracture bones, which can be devastating, particularly among the elderly.

Broken hips, for example, are the leading cause of nursing-home admissions in Canada and one of the leading causes of death. A broken hip is the precipitating factor in one in six deaths, research shows.

More than 1.4 million Canadians have osteoporosis, including one in four women and one in eight men older than 50. The cost of treating osteoporosis and the fractures it causes is an estimated $1.9-billion a year, according to the consumer group Osteoporosis Canada.

While the new research was conducted strictly on women, researchers said the treatment would probably be equally effective with men.