Ever since 1807, when Abraham Wood smoked opium to keep awake for a 24-hour race against Robert Allardyce, athletes have risked their health and professional careers by taking performance-enhancing drugs.

It seems that they need not have bothered. Most benefit derived from doping is in the mind, an Australian study suggests.

Over eight weeks at the Garvan Institute in Sydney, athletes were given either growth hormones, which are banned by the World Anti-Doping Association, or inactive placebos - without knowing which substance they were taking.

At the end of the study, volunteers who took placebos could sprint faster, jump higher and lift heavier weights.

Ken Ho, who led the study, said that the results showed the power of the mind in sport. “We found that athletes were putting their careers at risk by using growth hormone despite any evidence it actually improves performance,” he said. “We wanted to know if any improvement was due to the athlete's own belief.

“The results suggest the placebo effect was very powerful. If you really think you are receiving a beneficial treatment, you will perform better. Athletes who believe they are cheating gain an advantage even though they receive no chemical assistance.”

At the end of the experiment, the volunteers who were given placebos were asked what they thought they had swallowed. About half thought incorrectly that they had taken growth hormones while the other half guessed correctly they had taken the dummy drug. The volunteers who got it wrong outperformed everyone else in their group.

The placebo effect was greater in males than females, said Dr Ho, who presented the study to the Endocrine Society's convention in San Francisco. Volunteers of both sexes who thought they were taking drugs showed a markedly improved performance.

Although the study was carried out on “recreational athletes” who trained twice a week sports psychologists said there was no reason why the placebo effect would not apply to professionals. “The key aspect is confidence,” said Rob Robson, a sport psychologist. “If an athlete thinks he's taking a powder that gives him an advantage, he'll become more relaxed. He'll push himself harder and build confidence that improves his times.”

Legal experts disagreed on whether athletes who mistakenly thought that they were cheating could fall foul of anti-doping laws. Nick Bitel, who lectures on sports law at King's College London, said: “If someone told their trainer to buy them HGH growth hormone and then took it thinking it was HGH when it was actually sugar, they wouldn't have a defence.”

A spokesman for the World Anti-Doping Association said that the rules did not cover “placebo dopers”. He said: “If the substance involved is not a banned substance, that would not fall under the scope of anti-doping laws, regardless of any intention to cheat.”