The test was first reported on The Charlotte Observer's Web site.
A source close to Peppers, speaking on condition of anonymity, confirmed that report and said that steroids were not involved.
The rookie defensive lineman should be able to play Sunday against Tampa Bay while the NFL continues its investigation.
"My understanding is he will be our starting left defensive end this Sunday and all of my focus -- and his, too -- is on the Tampa Bay Buccaneers," said coach John Fox.
Panthers general manager Marty Hurney, citing league rules, would not comment on Peppers' situation. Marvin Demoff, Peppers' agent, did not immediately return calls from the Associated Press.
A second test on the initial urine sample will be done in the next few days, and if that comes back positive, Peppers would have 10 days to appeal the findings.
Peppers practiced with the Panthers (3-6) on Thursday, but did not make himself available to reporters.
Panthers linebacker Mark Fields said Peppers told him he plans to appeal if suspended.
"It's not that serious, so we're not looking at it like it's that serious," Fields said. "We're playing Tampa Bay Sunday, and he's playing."