Diaz made the accusations during an interview with Sportsnet Radio on Thursday following a UFC news conference at the Bell Centre.
"I believe [St-Pierre] is on plenty of steroids, and I don't think they test around here, either," Diaz said. "I doubt I'll be tested. I don't care what they're saying or marketing to the media -- and if so, he's probably got a bottle of p--- in his pocket. I doubt they're standing over him making sure he's not on steroids."
St-Pierre (23-2) will be seeking his eighth consecutive title defense against Diaz at UFC 158. The bout will be regulated by the Quebec Athletic Commission.
The Canadian welterweight hasn't suffered a loss since April 2007 and has never tested positive for a banned substance.
At Thursday's news conference, prior to Diaz's accusations, White downplayed any notion St-Pierre is or has ever used performance-enhancing drugs.
"He's never been busted for anything," White said. "The guy's fought a million times in title fights, and mostly guys in title fights are the ones that get tested."
St-Pierre made a similar comment during the news conference, stating, "I've never cheated once in my life." Diaz (26-8) has failed two drug tests in his career, both due to marijuana use. The Nevada commission suspended him one year after he failed a drug test following a decision loss to Carlos Condit in February 2012.
Diaz is a certified medical marijuana user in California, the state in which he resides. He appealed the commission's decision in Nevada court, a case he ultimately lost.
Marijuana is not a banned substance out of competition, meaning Diaz is free to use it between fights. He must, however, discontinue use ahead of a fight so that metabolites, a non-active byproduct of the drug, are clear of his system.
"I did a little more than I did last time," Diaz said. "I'm sorry if I don't pass the test, but I think it should work out."
White confirmed that a third positive test from Diaz at any point likely would result in his termination from the UFC.