Graphic: Running the risk - the possible side-effects of performance enhancing drugs

The problem with performance enhancing drugs is that there is such a small body of science to prove that they are bad for you. All there is, the scientists say, is anecdotal evidence.

Dwain Chambers made his third return to competitive sprinting in a meeting in Kalamata in Greece yesterday and he says that he is clean. The kind of anecdotal evidence that may haunt him, though, is the death in January 1993 of Jon Pall Sigmarsson, four times the “Strongest Man in the World”. Sigmarsson died of cardiovascular disease at 32 and is said to have been a steroids user.

Then there is the death on the Tour de France in 1967 of Tommy Simpson, who was 29 and had stimulants in his bloodstream. And between 1987 and 1990, a rash of Belgian and Dutch cyclists died from nocturnal heart attacks. In 2004, when Johan Sermon, a 21-year-old Belgian cyclist, died of a heart attack, he was the seventh such rider to die in this way in less than a year. All these cyclists' deaths were connected to the use of blood-doping products. This evidence comes under the heading “anecdotal”. The reason that our knowledge is incomplete, why there is no proof that drugs that are performance-enhancing can be life-threatening, is because no scientific examination has been given ethical approval. This is unlikely to change. Neither are we likely to witness a test to examine whether jumping off a cliff should be considered a health risk.

Science is scant on what quantities of drugs are acceptable to human physiology. When Linford Christie tested positive for nandrolone in 1999 he was said to be 100 times over the limit, but no scientist is allowed to experiment with a quantity so high.

So Chambers makes for interesting science. Some of the stuff in his body had been barely heard of before he tested positive. And what are the cumulative effects of “stacking” a shopping list of seven substances the likes of which he was on in 2003? Again, no one knows because no scientist would be given approval to test such stacking, although the closest we may have got to knowing may have been in a San Francisco courtroom in March 2004, when Kimberly Bell, a graphic artist from California, gave evidence to a Grand Jury about what it had been like being Barry Bonds' girlfriend.

Like Chambers, Bonds, the former San Francisco Giants baseball player who holds the all-time record for home runs, was a client of Victor Conte, the founder of the infamous Balco laboratory. Bell told the grand jury of the acne on Bonds's back, of his hair loss and of personality flips and furious rages. Bell said that she thought she had witnessed the enlarging of his head to such effect that she could see the plates of his skull. One juror asked Bell about Bonds's testicles. She replied that they had not gone for good but had shrunk.

Whether Chambers suffered in this way is probably a story for his autobiography, albeit one that he may sidestep. He did, however, stack all three substances that, separately, killed Simpson and were implicated in the deaths of Sigmarsson and the tragic peloton. If Chambers is 100 per cent clean, as he swears, he has avoided the immediate heart risks suffered by the cyclists. Indeed, what we can conclude from his apparent good health is how sophisticated was Conte's regime.

When Chambers gave his tell-all testimony to UK Sport three weeks ago and declared what drugs he had been taking, his admission to the use of liothyronine, a thyroid hormone, confused even the smartest experts. Liothyronine is not on the banned list and has never been considered a performance-enhancing drug.

As Professor Peter Hemmersbach, who runs Norway's IOC-accredited testing laboratory, said: “When we read what athletes are taking, we sometimes think, "What the hell would they use that for?'” And how will it damage them? For whether Chambers runs fast when he is apparently clean, even if he runs faster than in Kalamata yesterday, remember that some effects of steroids never leave him. A body that built muscle mass on drugs can retain it and thus retain its enhancing benefits.

Likewise, the body that has taken designer steroids such as tetrahydrogestrinone, better known as THG, or insulin may carry its side-effects for years. At the extreme we have East Germans who were doped from childhood and eventually suffered organ failure and impotence, side-effects that in some cases led to sex changes.

So who knows what be the state of Chambers's kidneys, liver, heart and arteries in years to come? He did not dope like an East German, but if you are cheating, where do you draw the line? No one knows because it is too dangerous to find out.