MANKATO, Minn. -- Minnesota Vikings running back Adrian Peterson said Friday he is certain that NFL players have used and are using HGH and that he is looking forward to the day when the league establishes a testing program to start "evening out the playing field."

Peterson acknowledged social media speculation that he used the drug to aid his recovery last year from knee surgery and said he is eager to prove he is clean.

"You've got HGH, something that doesn't show up on a test, and you've got guys out there trying to provide for their families," Peterson said. "They're going to try to get that edge, get that advantage, especially if they're not worried about trying to get caught. Yeah, it's being used."