Dr David Cowan, director of the UK Drug Control Centre at King's College, London, said: 'Sports competitors may have thought they would beat the testing system but they were wrong.'
Clenbuterol is not available on prescription in Britain, although it is prescribed in Germany for asthma.
It is also used for doping race horses in the United States and has been used illegally in Ireland to increase weight in beef cattle. Two Irish farmers who were accidentally exposed to the drug died of side effects.
'When clenbuterol is given for asthma minute amounts are used,' Dr Cowan said. 'It is prescribed in 10 to 20 microgram doses for asthma while athletes who take it have been using it in milligram quantities (that is a hundred times the prescribed dose), according to documents like the underground steroid handbook.'
Clenbuterol has two actions that athletes find helpful - it acts both as a stimulant and as an anabolic steroid.
As a stimulant it increases adrenalin production, the body's fight and flight hormone, enabling an athlete to endure and compete for longer periods of time.
But in doing so, it overrides the body's control mechanism which tells the athlete when he or she has had enough and should rest. As a result there is a danger of death following excessive exertion.
One of the particular advantages that clenbuterol may give an athlete is dilation of the windpipe and associated tubes in the lungs, enabling him to obtain more oxygen and so stretch one crucial limiting factor to performance.
The anabolic steroid effect of clenbuterol is probably very similar to that of methandienone, the steroid found in tests on Jason Livingston, who was sent home from Barcelona at the same time as the two weightlifters.
Methandienone is better known as Dianabol, its trade name when marketed by Ciba Geigy, the Swiss pharmaceutical company. It has not been made by Ciba Geigy since 1969 but is still made in India, the United States and Mexico, and is distributed in Europe on the black market.
Steroids stimulate muscle development in women and in boys. Adult males do not seem to develop increased muscle as a direct result of taking steroids. When male students are given the drug in double blind trials and follow a prescribed training schedule they do not develop more muscle than students given dummy tablets. However, athletes who take the drugs when training according to their own schedules do put on more muscle than athletes given dummy tablets. The reason for this is almost certainly the psychological effect of steroids, which are closely related to the male hormone testosterone.
Men become more aggressive and competitive when taking steroids. This enables them to push themselves further in training and so put on more muscle than when not using the drugs. They also help to give athletes an extra edge in competition. Women who take them develop extra male competitiveness but also risk developing the secondary male characteristics such as body hair and voice change.
Steroids also have numerous side effects. They can cause muscle cramps that interfere with performance and the accumulation of fluid which puts an extra load on the heart. This may increase blood pressure and so incur a risk of stroke in vulnerable people.
At the Drug Control Centre in King's College scientists now perform six broad screening tests on each sample of urine.
These tests, in effect, screen for hundreds of drugs. Samples that fail the test are then examined in more detail.
'We work to a very high standard,' Dr Cowan said. 'We must be absolutely certain that we do not accuse anyone falsely. Our findings have to be able to stand up in a court of law because any athlete who believed he was wrongly accused has recourse to the courts.'
However, the system is not yet entirely foolproof. There is still the possibility of athletes using drugs to assist in training between competitions, particularly in Germany and the United States where random testing is not yet fully accepted or implemented.
It is also possible to cheat by using the natural male hormone testosterone. The internationally accepted test for testosterone measures it with reference to another natural hormone, epitestosterone.
When testosterone is taken alone, the ratio of the two substances is distorted. However, it is possible to evade detection by taking epitestosterone as well. Dr Cowan can detect such evasion by measuring the proportion of testosterone in the body with reference to another natural substance, luteinising hormone, which cannot be mimicked.
'A competitor who is very smart might beat our tests but he would always be gambling,' Dr Cowan said.
'I don't think I could do it with absolute certainty and I know how the tests work.'