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CLENBUTEROL is a performance-enhancing drug that is banned by the World Anti-Doping Agency. 

It is not approved for human use in Australia. 

Collingwood players Lachlan Keeffe and Josh Thomas are facing potential two-year bans after they tested positive to the drug on February 10.

The drug is well-known in professional sport and rose to prominence in 2010 when disgraced Spanish cyclist Alberto Contador tested positive during the Tour de France. He was subsequently stripped of his title and banned for two years.

The Contador saga is the most infamous case of the drug's use in world sport, but clenbuterol has also been used in bodybuilding to improve muscle strength and reduce body fat, according to renowned footy medico Dr Peter Larkins.

"It's well-known in the sporting world. This product has been misused in sport for a couple of decades because it is an anabolic agent," Larkins told AFL.com.au

"It's a veterinary product, so it's not approved for human use. But it is an asthma product used particularly for horses, or equine use.

"There can be some problems with asthma patients in the Olympics having to get permission to use their asthma drugs because there's a crossover effect on positive testing.

"This particular one is banned by WADA and the IOC because it's a definite anabolic agent.

"So as well as being an asthma product it improves muscle strength and it reduces body fat."

Contador's defence centred around eating contaminated meat. He failed, but Australian cyclist Michael Rogers successfully used the same defence when he was cleared of a positive clenbuterol test recorded in Japan in October 2014, claiming he ingested the drug in contaminated meat consumed in China during the Tour of Beijing earlier that year. 

The drug dilates the bronchial system and its effects can last 24-48 hours. 

Because the substance can't be prescribed to humans in Australia, Larkins said the only ways to source it were the black market or veterinary supply. 

"It's not something you just walk up to a chemist and get, so you've got to have pretty good contacts to get hold of it," he said.

"I obviously don't know the Collingwood circumstances in detail to know how it got into their bodies, but you'd be pretty dumb if you were trying to use a product like that to prepare yourself for the season because it's such a banned substance."

Keefe and Thomas face a minimum of a two-year ban if their 'B' sample tests come back positive and they are found guilty of breaching the AFL's anti-doping code.

"It's a two-year ban for using an anabolic product not approved for human use," Larkins said.

"It's very clear in the WADA and ASADA code for this type of product."