Greek weightlifter Leonidas Sampanis has been stripped of his bronze medal after testing positive for twice the allowed amount of testosterone.
Sampanis took bronze in the men's 62kg category
Nine weightlifters have failed drug tests but Sampanis' is the first medal to be taken away from an athlete.
Sampanis, who won bronze in the 62kg category on Monday after taking silver at the previous two Olympics, maintained his innocence.
"I deserve this medal," he said. "I won't give it back."
Doctors established the testosterone came from outside the body, although Sampanis denies any wrongdoing.
Patrick Schamasch, IOC medical director, said: "We did receive a therapeutic
use exemption (TUE) from Sampanis because he had an injection in his knee.
"This question was raised...as to whether this would have influenced
the result. There is no way that this could have happened.
"The ratio was over twice the legal permitted amount and, under strict
liability, there is nothing more to discuss.
Sampanis said on Saturday: "Honestly, I can say that for the last 10 years, the time that I have been a weightlifter, I never used this kind of drug," said Sampanis.
"I want to say to the Greek people that I swear to God and on my two children I never used this kind of drug.
"I want you to believe me. I want you to stand by me."
Greece's two biggest track medal hopes, Kostas Kenteris and Katerina Thanou, were withdrawn from the Games after they missed a drugs test in mysterious circumstances just before the Games began.
The Greek government vowed on Friday to crack down on any athlete who tests positive FOR banned substances.
"For any athlete who has taken banned substances the reaction of the government will be very hard," a government source said.
"They will be stripped of any medals, awards or other privileges according to Greek laws."
The weightlifitng competition in Athens has been dominated by failed drug tests.
Russia's Albina Khomich became the ninth lifter to test positive on Saturday.