Taking prescription drugs to prevent diabetes cannot be justified when lifestyle changes are just as effective, say US scientists.
Currently, drugs are given if lifestyle changes fail
Trials show rosiglitazone pills do work, but so does weight loss, which is a safer and cheaper way to avoid type 2 diabetes, the researchers argue.
In the British Medical Journal, they warn about turning healthy people into patients and the risk of side effects.
The charity Diabetes UK also strongly advocates lifestyle change methods.
Over two million people in the UK are diagnosed with type 2 and up to 750,000 more are thought to be undiagnosed.
Type 2 diabetes is linked to lifestyle factors such as obesity and experts believe up to half of all cases could be prevented through changes to diet and exercise.
Currently, drugs are given if lifestyle changes fail to control blood glucose.
In a study published in The Lancet medical journal last September, experts suggested that one in 12 people should be taking rosiglitazone to prevent the disease.
And they said giving rosiglitazone to people with impaired glucose regulation would prevent one in seven cases.
But Professor Victor Montori and colleagues, from the Mayo Clinic College of Medicine, Rochester, questioned the findings of studies looking at using drugs to prevent diabetes.
They argue in the BMJ: "If clinicians offer patients glitazones to prevent diabetes, they are offering certain inconvenience, cost, and risk for largely speculative benefit.
"Lifestyle changes are clearly at least as effective as glitazones and can be implemented considerably more cheaply."
There is also a danger that offering drugs would send out the wrong message to people - that they could reduce their risk without making lifestyle changes.
And it would cost the NHS money, although it would also potentially save it money by reducing the health complications associated with diabetes.
Simon O'Neill from Diabetes UK said: "Much more research needs to be done to justify the use of this drug in preventing Type 2 diabetes.
"We strongly recommend that people at risk of developing Type 2 diabetes should be incorporating increased levels of physical activity into their daily lives alongside making changes to their diet."