A drug used to treat diabetes also seems to prevent it in people at high risk of developing the disease, the largest study ever to test this has found.

The drug, rosiglitazone, or Avandia, appeared to cut the risk of developing diabetes or dying by more than half in the study. It also helped restore normal blood-sugar function in many participants.

A second part of the study found that a different drug, a blood pressure medication called ramipril, or Altace, made no difference in the risk of developing diabetes but helped normalize blood sugar for some.

The research was long-awaited, and the Avandia results at first glance seem impressive, but experts say it is difficult to determine how much impact the drug had, because study volunteers also were regularly counseled about healthy diets and lifestyles.

"We know that lifestyle changes alone can reduce the risk of developing diabetes by up to 58 percent," said Dr. Martin Abrahamson, medical director of the Joslin Diabetes Center in Boston, who had no ties to the study.

Those benefits come without the $100-a-month cost and side effects of Avandia, said Dr. Alvin Powers, director of diabetes research at Vanderbilt University Medical Center who also had no role in the research. He noted that a small percentage of those on Avandia developed heart failure.

Type 2 diabetes, the most common form and the type that is linked with obesity, is a growing problem worldwide. An estimated 220 million people around the world and 18 million Americans have the disease, which can lead to kidney failure, amputations and death. In Type 2 diabetes, the body does not make enough insulin or cannot effectively use the insulin it manages to produce.

Results of the drug study were to be presented Friday at a diabetes meeting in Denmark. The Avandia findings were published online by the British medical journal The Lancet; the Altace results were posted online by the New England Journal of Medicine. Both will appear in print editions later.

The study was paid for by the Canadian Institutes of Health Research and companies that make the drugs (GlaxoSmithKline PLC makes Avandia; Sanofi-Aventis SA and King Pharmaceuticals market Altace). Some study leaders consult for the drug companies.

The study involved about 5,000 people with "pre-diabetes," or blood-sugar abnormalities. Research suggests that as many as half of them will develop Type 2 diabetes within three years.

Doctors at McMaster University in Canada and in 20 other countries assigned these pre-diabetics to get either Avandia, Altace, both drugs or no drug.

Results for the combination treatment were not presented.

In the Lancet study, 306 of the 2,365 people given Avandia for an average of three years developed diabetes or died, compared with 686 of the 2,634 who did not receive the drug.

Fourteen of those given Avandia developed heart failure, while only two cases of heart failure occurred in people who didn't take the drug. Some doctors believe the heart risk is manageable as long as physicians carefully check patients taking the drug for heart abnormalities.

However, Powers said a drug to prevent one disease β€” diabetes β€” must not bring a substantial risk of another, or doctors will be unwilling to prescribe itβ€” especially when lifestyle changes and other drugs such as metformin can prevent diabetes, too.

Still, some doctors were encouraged by Avandia's potential.

"This underscores the fact that diabetes is preventable, and that we might have another means to do that with," said Dr. Peter Sheehan, director of diabetes at the Cabrini Medical Center in New York, who had no ties to the study.