Expulsion of an American judo player from the London Olympic Games after he tested positive for marijuana prompted scientists to question  the drug's inclusion on the World Anti Doping Agency's (WADA) banned list.Few experts think marijuana, or cannabis - whether it's eaten or smoked - can do much to enhance the kind of speed, strength, power or precision that Olympic athletes strive for.

And many wonder whether the expensive time and effort of sporting drug testers might be better spent catching serious cheats who top up their blood with EPO or pop anabolic steroids to boost testosterone levels and muscle growth.

"There's no evidence cannabis is ever performance enhancing in sport, and since its use is legal in a number of countries, there's no reason for it to be banned by WADA," said David Nutt, a professor of neuropsychopharmacology at Imperial College London.

"I can't think of any sport in which it would be an advantage. And it seems ludicrous that someone could quite legally smoke cannabis in Amsterdam in the morning and then come over to London in the afternoon and be banned from competing."

The heart of the problem is where to draw the line between performance enhancing drugs - which many experts agree should be prohibited in sport because they make the contest unfair - and recreational drugs like marijuana, which is unlikely to boost performance but could give sport a bad image.