Doctors believe that an increasing number of bodybuilders are putting themselves at risk by injecting themselves with insulin.
It is easy to overdose on insulin
The drug, normally given to diabetics to help control their blood sugar levels, is highly effective at helping bodybuilders boost their muscle mass.
However, this comes at a price - careless use could cause a plummet in blood sugar levels which could leave the bodybuilder at risk of coma, or even death.
Over the long term, the extra insulin could cause irreversible long-term damage.
Bodybuilders who use insulin could end up becoming diabetic themselves as their body's natural mechanism for producing the hormone stops working properly, warn experts.
The problem was highlighted by Dr Richard Lynch, an A&E doctor from Pontefract General Infirmary in Yorkshire, who encountered a bodybuilder who was found unconscious at home.
His symptoms matched those of a diabetic who had experienced "hypoglycaemia" - dangerously low blood sugar.
However, as he came round it was revealed that he was a bodybuilder who had been taking insulin to improve his muscle bulk.
"Normal" practice among bodybuilders taking insulin is to combine the drug with a large amount of sugary food to stop blood sugar levels dropping too low.
However, because the 31-year-old was close to a competition, he had, in addition, been on a strict diet, which compounded the problem.
Although the man later recovered fully, Dr Lynch offered this warning to other bodybuilders.
He said: "This potentially lethal drug has serious consequences should things go wrong, particularly as it is usually used in secret.
"This puts the user at risk of developing hypoglycaemia for prolonged periods away from possible medical assistance, potentially resulting in coma and death."
The precise source of the insulin is unknown, although it is rumoured that some diabetics sell some of their prescribed insulin to bodybuilders.
A spokesman for Diabetes UK said that it was particularly dangerous for a non-diabetic to take insulin.
She said: "These people already have enough insulin. What they are doing is extremely risky.
"It's extremely easy to overdose on insulin if you haven't been trained in what to do."
She said that the training process for newly-diagnosed type I diabetics was, at least at first, normally carried out in hospital by experienced diabetes doctors and specialist nurses to gauge exactly how much insulin each patient needed.
Insulin abuse is also suspected in professional sport as an alternative to anabolic steroids - which can be detected by testing.
In contrast, an insulin injection is virtually undetectable.
The study was published in the British Journal of Sports Medicine.