Gay, who had the year's fastest 100m time of 9.75 seconds, said in a telephone conference call to two reporters that he was notified by the US Anti-Doping Agency on Friday that his A sample from an out-of-competition test on May 16 had returned a positive.
"I don't have a sabotage story. I don't have lies...I basically put my trust in someone and I was let down," said Gay, who added he had never knowingly taken a performance-enhancing drug.
"I made a mistake," added the 30-year-old sprinter from his training base in Amsterdam.
"I am pulling out of Monaco (Diamond League meeting on Friday) and the world championships."
The B sample is yet to be tested, said Gay.
Gay, the world 100 and 200 champion in 2007, said he could not divulge the substance or how the positive occurred.
"I am not allowed to talk about those things right now.
"I know exactly what went on, but I can't discuss it right now," he added.
Healthy after years of injury, Gay, the joint second-fastest man in history over 100m, had been considered a strong challenger for Jamaican world record holder Usain Bolt at the August 10-18 world championships after winning the American trials in blazing times.
He had put down the year's three fastest 100m times, topped by his 9.75 at last month's US meeting. Only Bolt has run faster than Gay's 19.74 seconds in the 200m this year.
"It has been tough for me," said the soft-spoken Gay. "I have always been a drug-free athlete."
USA Track and Field, the sport's US governing body, said in a statement: "It is not the news anyone wanted to hear, at any time, about any athlete.
"As we approach the world championships, we will remain focused on the competition at hand and winning the right way."
The governing body said it did not know the facts of the case and looked to USADA to adjudicate it and handle it appropriately.
Gay appeared set to win his first Olympic individual gold medal at the 2008 Beijing Games but the emergence of Bolt and injuries ended that chance.
He ran an American record 9.69 seconds, at the time the second fastest ever, in 2009 but was never fully healthy at ensuing Olympic and world championships.
He finished a heart-breaking fourth at the London Olympics but won a silver medal in the 4x100m relay.
"I hope I am able to run again," he said. "But I will take whatever punishment I get like a man."
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