Merritt has dominated the 400m in recent years
Olympic and world 400m champion LaShawn Merritt failed three drugs tests between October 2009 and January 2010.
His lawyer Howard Jacobs claimed in a statement that the positive tests were caused by Merritt's "use of an over-the-counter male enhancement product".
Jacobs said: "LaShawn Merritt has accepted a provisional suspension as a result of [the] positive drug tests."
The American, 23, faces a two-year ban and admitted he had made a "foolish, immature and egotistical mistake".
Merritt claimed he had not been aware that the product he used contained the banned steroid Dehydroepiandrosterone (DHEA).
He said in a statement: "To know I have tested positive as a result of a product I used for personal reasons is extremely difficult.
Thanks to his selfish actions, he has done damage to our efforts to fight the plague of performance-enhancing drugs in our sport
UST&F chief executive Doug Logan
"I hope my sponsors, family, friends and the sport itself will forgive me. Any penalty I may receive for my action will not overshadow the embarrassment and humiliation I feel.''
In the statement, Merritt admitted he had not read the "fine print" on the product he purchased.
The Los Angeles Times quoted Doug Logan, the chief executive of USA Track & Field, as saying that Merritt had "sullied his career".
"He has put an unfortunate stigma on himself he is going to be living down the rest of his life."
Logan said Merritt's actions "indicate an extraordinary lack of maturity and an absence of the responsibility necessary to be a world-class athlete".
He added: "Thanks to his selfish actions, he has done damage to our efforts to fight the plague of performance-enhancing drugs in our sport."
The runner's claim that he was unaware the product contained DHEA is unlikely to help him avoid a ban as anti-doping rules state athletes must take ultimate responsibility for what is found in their bodies.
Merritt, who upset favourite Jeremy Wariner to win the Olympic 400m title in Beijing two years ago, was not notified of the positive tests until March.
As well as winning individual gold at the 2008 Beijing Olympics and Worlds in Berlin a year later, he also helped the US team win 4x400m relay titles at both events.