BBC's Pallab Ghosh: "Many are calling for more research"

Some banned substances, such as stimulants like cocaine and artificial steroids, are relatively easy to deal with because they are not found naturally in the body. If these are detected at all, the athlete is banned.

Nandrolone, a close chemical cousin of testosterone, was thought to be in this category up until this year. But a study has since shown that normal people can have a small but significant level in their bodies - 0.6 nanograms per millilitre of urine.

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The International Olympic Committee have set a limit of two nanograms per millilitre. But expert Dr Mike Wheeler, at St Thomas' Hospital, London, told BBC News Online that this is "awfully close" to the level at which an unnacceptable number (usually more than 0.01%) of innocent athletes might produce positive tests.

These false positives might be explained in a number of ways. Firstly, the stresses athletes place on their bodies in training and competing could raise the natural levels of banned substances.

"There is some evidence that after an event, an athlete's testosterone goes up," said Dr Wheeler. "Therefore if nandrolone is produced from testosterone, it could be that the nandrolone goes over the detection limit."

"We have to remember that the nandrolone studies used very few individuals and, crucially, not athletes."

Food supplements

Testing for steroids is best done on urine

The other major problem is that some food supplements contain allowable steroids which the body readily converts into the banned nandrolone.

"I went out and bought some in a sports shop this morning," said Dr Wheeler. "Therefore, athletes may in all innocence be taking supplements which contain steroids they don't know about but which will result in a positive test."

Furthermore, there is some suggestion that nandrolone can be found in meat injected with steroids, or vitamins. In women, the presence of nandrolone may be due to pregnancy or the use of some contraceptive pills.

Michelle Verroken of UK Sport says: "It is important to try to get to the bottom of this. Are we seeing athletes using supplements they haven't known the outcome of? Or is it due to the change in the way animals are being farmed? Or are we seeing athletes actually using nandrolone as a substance to increase their performance?"

True test

Dr Wheeler says there is definitely a solution. The recent work has shown that naturally-occurring nandrolone breaks down in the body to a number of smaller molecules. The ratio of these break-down products is "profoundly different" in people who have taken nandrolone deliberately.

"But more, larger experiments are required as the study only used 30 normal people," he said. "And most research is lengthy."

The way it breaks down mean nandrolone is actually rather easier to detect than some other banned substances which also occur naturally in the body, such as growth hormones, which boost muscles.

Professional cyclists have used EPO

Erythropoietin, or EPO, has been another troublesome performance enhancer. It stimulates the production of red blood cells, meaning more oxygen can be carried, but can fatally thicken the blood.

It has been used for many years in endurance sports but a reliable test was only developed in June 1999.