The largest diabetes prevention study ever done has found that a drug already used to treat the disease also can help keep “prediabetics” from developing it. But many experts say that losing weight and exercising remain a safer, cheaper approach.

The drug, rosiglitazone, or Avandia, appeared to cut the risk of developing Type 2 diabetes by more than half, doctors reported Friday. Type 2 is the most common form of diabetes and a public health menace that afflicts more than 200 million people worldwide.

Avandia also helped restore normal blood-sugar function in many of those who took it.

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. However, experts say it is difficult to determine how much of the improvement was due to the drug, 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 $90- to $170-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.

“Fluid retention, congestive heart failure and weight gain are known side effects of Avandia” when it’s used to treat diabetes, Powers noted.

Results of the study were reported Friday at a diabetes meeting in Denmark.