Diabetics are selling their life-saving insulin to bodybuilders who use the drug as a performance booster.

The black market in the diabetes drug is revealed in the British Journal of Sports Medicine, which says nearly one in 10 bodybuilders may be using insulin to increase stamina and muscle tone. Doctors warned that bodybuilders risk serious injury and even death by taking insulin.

Dr Richard Lynch, an A&E consultant at Pontefract General Hospital in Yorkshire, investigated the problem after he treated a 32-year-old bodybuilder who collapsed into a coma after taking insulin.

"The man was brought into casualty unconscious last year," Dr Lynch said. "We assumed he was a diabetic who had overdosed but when he came round he admitted he was a bodybuilder and had been taking a friend's insulin to enhance his performance. He said it was a common practice.

"Even after collapsing in a coma, he was not going to stop using insulin."

Insulin is available only on prescription but the regulations allow diabetics to access repeat supplies over the counter at pharmacies. Dr Lynch said: "Some diabetics are buying more insulin than they need and selling it on. It is a secretive process, so we don't know how much it is being sold for, but we know it is going on."

One fitness instructor told Dr Lynch that 10 per cent of the bodybuilders he knew were taking insulin. It helps to build muscle by increasing the storage of carbohydrates and amino acids. If taken with glucose, or sugary food and drink, it also increases energy supplies and improves stamina.

The prescription insulin is undetectable from naturally produced insulin, so anti-doping tests at contests will not pick it up. But doctors have warned that insulin use can be fatal for non-diabetics. It can take so much sugar from the blood that the brain is deprived of oxygen, causing coma, brain damage and death.