A Greek Olympian has been stripped of his bronze medal after testing positive for drugs, and a Russian gold medallist has also failed a test.
Greek weightlifter Leonidas Sampanis tested positive for twice the allowed amount of testosterone.
Russian shot putter Irina Korzhanenko was found to have taken steroids and is expected to lose her medal and be sent home, officials said.
Cuban silver medallist Yumileidi Cumba is in line to take Korzhaenko's place.
Nine weightlifters have failed drug tests but Sampanis is the first athlete to have a medal taken away.
Sampanis took bronze in the men's 62kg category
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Greece's two biggest track medal hopes, Kostas Kenteris and Katerina Thanou, were withdrawn from the Games after they missed a drugs test in mysterious circumstances just before the Games began.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.
Shot put 'tarnished'
The shot put competition was held at Olympia, the birthplace of the Olympics, and was meant to be a showpiece of the Games.
"It's very sad and it tarnishes what was intended to be a very symbolic event," AFP quoted Russian Olympic Committee spokesman Gennady Shvets as saying.
Korzhanenko, who dominated the competition, was stripped of the 1999 World Indoor title for drugs offences.
Uzbek women's shot putter Olga Shchukina was expelled from the Games on Friday after failing a drugs test.
And Yanina Korolchik of Belarus, Olympic champion in Sydney, is currently serving a drugs ban.