Judo fighter Nicholas Delpopolo has tainted the US team image after he tested positive for cannabis.
However, he claimed that he'd ingested marijuana accidentally after it was baked into something that he ate before he set off for London.
He said in a statement: "[the positive test] was caused by my inadvertent consumption of food that I did not realise had been baked with marijuana."
He added: "I apologise to U.S. Olympic Committee, to my teammates, and to my fans, and I am embarrassed by this mistake. I look forward to representing my country in the future, and will rededicate myself to being the best judo athlete that I can be."
The International Olympic Committee said it had thrown the competitor out of the 73-kilogram class after testing positive for the drug on 30 July.
In a statement emailed to The Huffington Post UK, the IOC said: "The International Olympic Committee today announced that it has disqualified American judoka Nicholas Delpopolo from the men's 73kg judo event of the Games of the XXX Olympiad in London.
"Immediately after his participation in the event on 30 July, Delpopolo, 23, provided a urine sample that tested positive for the prohibited substance 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid."
The IOC added: "Mr Nicholas Delpopolo is disqualified from the men’s 73kg judo event of the 2012 London Olympic Games where he placed 7th; shall have his diploma in the above-mentioned event withdrawn; and shall have his Olympic identity and accreditation card cancelled and withdrawn immediately."
The fighter is ranked No1 in the US and sixteenth in the world.
On his website he says that his guilty pleasures are "Online Gaming, Yu-Gi-Oh!, Magic the Gathering, “The Jersey Shore”, Staying up all night, looking in the mirror".
Delpopolo is the fourth athlete to be excluded from the Games, while one other has been provisionally suspended.
Weightlifter Hysen Pulaku from Albania tested positive on 23 July for stanozolol, Uzbek artistic gymnast Luiza Galiulina was excluded after testing positive for the prohibited substance furosemide, Russian cyclist Victoria Baranova was excluded after testing positive for taking testosterone and Colombian 400m runner Diego Palomeque Echavarria has been provisionally suspended.