American track star LaShawn Merritt tested positive for a banned substance used in an over-the-counter male enhancement product, an embarrassing episode for an Olympic gold medalist who provided one of the best feel-good stories in Beijing.

Merritt, who overcame his rival Jeremy Wariner to win the 400-meter gold at the Olympics, accepted a provisional suspension Thursday, agreeing to sit out while the U.S. Anti-Doping Agency resolves his case.

It's a humiliating come-down for one of track and field's rising stars over the past two years. And the 23-year-old Merritt said he hopes his family, friends and sponsors will forgive him for making "a foolish, immature and egotistical mistake."

According to a release from Merritt's lawyer, Howard Jacobs, the sprinter used a male enhancement product containing substances that caused him to fail three successive tests between October 2009 and January 2010.

If that's the extent of his positive tests, it would presumably not put his Olympic medals or his 2009 world championships at stake. Merritt won gold in the 400 at 2009 worlds in Berlin and teamed with Wariner to win the 1,600-meter relay in Beijing and Berlin.

Merritt's reputation could suffer a hit, though, and already his standing at USA Track and Field has been tarnished.

USATF's CEO, Doug Logan, said in a release he is "disgusted by this entire episode."

"He has now put his entire career under a cloud and in the process made himself the object of jokes," Logan said. "In this day and age, a professional athlete should know better."

Merritt recently was notified that the presence of DHEA was the cause of his positive test. DHEA is short for dehydroepiandrosterone, a steroids precursor on the World Anti-Doping Agency's banned list.

The product that Merritt took is called "ExtenZe," a person familiar with the case told The Associated Press. It's a product that promises male sexual enhancement.

The person spoke on the condition of the anonymity because the details of the case were supposed to remain confidential.

USADA confirmed the sprinter had tested positive and had agreed not to compete until his case has been resolved.

The USADA process can last from weeks to months. Typically, an athlete guilty of a first-time doping violation receives a two-year suspension. USADA has a precedent of giving more favorable terms to athletes who cooperate.

"It's hard to say at this point what the end result would be," Jacobs said in a telephone interview. "But I think we can put forth a compelling case, and that it was inadvertent and the circumstances that led up to it."

Merritt said in a release that, "any penalty that I may receive for my action will not overshadow the embarrassment and humiliation that I feel inside."

Following his win in Beijing, he was given the red-carpet treatment in his hometown of Portsmouth, Va., where a parade was held in his honor.

There, Merritt had a chance to honor his late brother, Antwan, who died a decade ago, after falling from a dorm room window at Shaw University. Merritt has said his brother was the impetus for his success.

Merritt had an electric season in 2008, finally breaking through against Wariner. He snapped Wariner's nine-race winning streak at a meet in Berlin, a result that turned heads simply because nobody had really challenged Wariner since he won the Olympic gold in 2004.

Merritt went on to beat Wariner at the Olympic trials and again in Beijing.

Like that, their robust rivalry was on.

It has developed into one of the best in track, especially since nobody can touch Usain Bolt in the 100 and 200.

Last summer at the world championships in Berlin, Merritt topped Wariner again, flying past him on the final curve.

The latest news ends a period of relative quiet on the doping front in track. High-profile doping convictions in recent years have resulted in stripped results and suspensions for Marion Jones, Tim Montgomery and Justin Gatlin, among others.

"You've got to look at the facts of the case. This is different," said Jacobs, who represents many athletes in doping cases, including Jones and Montgomery. "In LaShawn's case, at the end of the day, I'd be surprised if anyone disputes what caused it. It's important that people try to recognize the difference and not lump them all into the same category."

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AP National Writer Eddie Pells contributed to this report.