"I believe that he is on plenty of steroids and I don't think they test around here either," Diaz said in an interview with Sportsnet Radio on Thursday. "I doubt I'll be tested as well. I don't care what they're saying or marketing to the media. I don't think either of us are going to be tested. And if so, he's probably got a bottle of piss in his pocket. I doubt they're standing over him and making sure he's not on steroids."
The California fighter faces St-Pierre on Saturday in the main event of UFC 158 at the Bell Centre.
Unlike St-Pierre, Diaz has failed drug tests. He is coming off a one-year suspension for his second positive test for marijuana.
Diaz says he used the drug medicinally to deal with social anxiety issues.
Diaz also raised the issues of cheating at Thursday's pre-fight news conference when he talked about "the strong guy on steroids" benefitting under the current MMA rules in terms of wrestlers holding opponents down.
The doping issue was raised again when the two fighters got involved in a verbal exchange.
"I'm not on steroids," said St-Pierre. "I've never cheated in my life."
"We'll see if they test who for steroids or what," answered Diaz.
"I hope they do," answered the champion.
UFC president Dana White also rubbished the drugs claim.
"He's never been busted for anything," he said of GSP. "The guy's fought a million times in title fights and mostly guys in title fights are the ones that get tested. He's never tested positive for anything even close."
In 2009, B.J. Penn also accused St. Pierre of taking steroids, although he said he had no proof.
"It's just my opinion that he uses steroids," Penn told Yahoo Sports. "That's it. My opinion. I do believe it. I can't hand you any proof, but that's my opinion."
Penn also accused St-Pierre of illegally greasing during their last fight. Again it was denied.
The Quebec commission will handle drug testing for UFC 158.
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