Caffeine is no longer a banned drug
British athletes routinely use caffeine to boost their performance, say researchers.
A third of track and field athletes and 60% of cyclists reported taking caffeine before competing, a Liverpool John Moores University study found.
The drug was removed from the list of banned substances in 2004 but its use is still monitored.
The study's authors said it raised concerns that athletes were exploiting caffeine to gain an advantage.
In recent years there have been widely publicised fears about increasing doping in sport and Beijing officials are planning on doing 25% more drug tests than were done in Athens.
Study leader Dr Neil Chester said the World Anti-Doping Agency had not been clear about why caffeine had been removed from the list but he understood it was because it was too hard to distinguish between normal social use and abuse of the drug.
A total of 480 athletes were questioned for the study, through athletics or cycling clubs and at sporting events.
They reported using caffeine in the form of energy drinks, sports supplements, pills and coffee, the International Journal of Sports Medicine reported.
Elite athletes were more likely to use caffeine to improve their performance than those who took part in sport recreationally.
Dr Chester said caffeine had been shown to be beneficial for endurance events and would also increase the alertness of athletes.
He added that the fairly recent introduction of caffeine to energy and sports drinks had increased the opportunity for athletes to consume the drug in high doses.
"There's been a lack of communication from WADA and there is a question about whether or not sporting authorities are condoning its use," he said.
"Ultimately there is a need to clarify the use of caffeine within the present anti-doping legislation."
Mark Stuart, who was a pharmacist for the Sydney Olympics and who has just written an editorial on the topic for BMJ Clinical Evidence, said there was an "obvious difference" between the view of the authorities and that of the athletes on the benefits of caffeine.
"There still seems to be some scope for athletes to exploit commonly available dietary supplements, such as caffeine, with minimal consequence."
Dr Samantha Stear, national nutrition lead at the English Institute of Sport, said there had been a lot of confusion among athletes about caffeine use which had been restricted but then moved to the monitoring list because caffeine was present in so many foods.
"Some find it beneficial and some don't, it's very dependent on the individual.
"We try and work with the minimal amount that's needed for the athlete."
A spokesman for the World Anti-Doping Agency said research suggested that caffeine actually damaged performance when used in significant quantity.
In addition, as caffeine was metabolised at different rates by different people, there was a danger that athletes could face sanctions simply for social consumption.