A wide range of drugs are available over the internet
It follows the case of a man who suffered repeated blackouts after taking an anabolic steroid that was withdrawn from use in 1982.
An article in the British Journal of Sports Medicine said the 30-year-old man, from the UK, had bought this and another substance on the internet.
He admitted buying and taking Dianabol, which was removed from the market 20 years ago because of its reported side-effects.
Reported side-effects of bromocriptine include low blood pressure, heart failure, aggravation of angina and heart attack.
During his stay in hospital, tests showed he had an abnormal heart beat and rate.
Although he was not taking a high dose of bromocriptine, the report's authors suggest that a strict diet and high doses of steroids probably exaggerated the drug's effects.
The easy availability of prescription-only drugs on the world wide web is cause for concern, particularly if they have already been officially withdrawn from use, said Dr Ganesh Manoharan, of the Royal Victoria Hospital in Belfast, who examined the patient.
Protein-rich diet
The body builder said both amateur and professional bodybuilders often use bromocriptine during the last few weeks of a training programme to burn off excess fat and confirmed that the drug is easily available on the internet.
A year later he had stopped taking bromocriptine, but continued taking anabolic steroids against medical advice.
He had also started on growth hormone and insulin injections.
Dr Manoharan said: "Worrying is the extent to which body builders abuse prescription-only drugs and the ease with which these and many other drugs are available to the general public through the internet without prescription."
His concerns are endorsed by Dr Paul Cundy, the British Medical Association's adviser on computers in general practice.
He said: "I have come across websites where you can buy any pharmaceutical compound, including heroin.
"Educating the public is the issue here.
"These drugs are controlled for a very good reason, because they are potentially damaging."
The National Amateur Body Builders Association (NABBA) said it had not heard of anyone buying banned substances over the internet, but would take a hard line with any members who did.
NABBA spokesman Paul Jeffreys said: "You don't need to take any false substances if you eat lots of protein on a high protein diet.
"All they're doing is cheating themselves if they're taking other things."