* Judo player expelled after eating marijuana brownie
* Scientists question reasons for banning cannabis in sport
* Little evidence to suggest it could enhance performance (Adds more detail on cannabis and sports, no comment from WADA)
By Kate Kelland
LONDON, Aug 6 (Reuters) - The expulsion of an American judo player from the London 2012 Olympic Games on Monday after he tested positive for marijuana prompted scientists to question the sense behind 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.
While it is generally accepted that cannabis is unlikely to give athletes any advantage in fast-paced sports, some experts say it could prove helpful in sports like shooting or golf where a steady hand is needed.
Under WADA's rules, athletes face a two-year ban if cannabis is found in their system while they are in competition.
But the anti-doping body does not sanction those who test positive for marijuana outside of competition times, while they are in training camps or during rest periods.
Scientists say this smacks of double standards and suggests WADA bans cannabis for political rather than scientific reasons.
"The problem is the elite athletes should be seen as role models for young kids, and so they ban cannabis because they don't want to have the image of gold medallists smoking joints," said one British-based sports scientist who asked not to be identified because of the sensitivity of the issue.
A photo of the American swimming champion Michael Phelps inhaling from a glass pipe used for smoking marijuana in 2009 sparked criticism from the U.S. Olympic Committee.
In a statement released shortly after the picture was published by a British tabloid newspaper, Phelps admitted to smoking pot and apologised for what he described as "bad judgment". He did not receive a doping ban because it was not "in competition".
Experts say that row, as well as Monday's ruling on American judoka Nick Delpopolo - who said he inadvertently ate the drug in a marijuana brownie - is far more to do with the image of sport than any form of cheating.
"It's hard to imagine how smoking a joint or eating marijuana brownies is going to help somebody in judo," said Michael Joyner, a member of the Physiological Society and a researcher at the Mayo Clinic in Minnesota in the United States.
"My advice to WADA is that they should focus on drugs that are clearly performance enhancing in the sports where they are clearly performance enhancing."
Some national sporting bodies are also kicking back against WADA's stance.
Australia's Coalition of Major Professional and Participation Sports called in May for marijuana to be removed from the list saying it was wrong to group it with performance enhancing drugs like human growth hormone and steroids.
Substances on WADA's banned list should meet two of the following criteria: they are proven to be performance enhancing, they are dangerous to the health of athletes, or they are contrary to the spirit of sport.
While there are few signs that marijuana can enhance sporting performance, there is evidence to suggest it could have a negative impact.
Studies have shown that THC - the ingredient in cannabis that induces the "high" - increases blood pressure and heart rate while also decreasing cardiac stroke volume, leading to diminished peak performance.
It can also slow reaction times, cause problems with coordination, reduce hand-eye coordination, and interfere with visual perception.
Anti-doping authorities were not keen to discuss the issue on Monday.
Officials at UK Anti-Doping declined to comment, and when Reuters sent emails to WADA's media relations office asking for a statement on why cannabis is banned, WADA responded by saying it was too busy to provide a comment on Monday.
WADA president John Fahey indicated in May the agency may look at changing the criteria for cannabis as a banned substance for athletes, but no decision is expected this year. (Editing by Ossian Shine/Greg Stutchbury)