Russian biathlete Olga Pyleva has been thrown out of the Games after becoming the first athlete to fail a drugs test.
The 30-year-old, who won silver in the 15km biathlon on Monday, tested positive for the stimulant carphedon.
Nearly 400 drugs tests have been conducted since 31 January, with Pyleva the only athlete to fail.
Pyleva has been stripped of her medal, with Germany's Martina Glagow moving up to silver and fourth-placed Russian Albina Akhatova taking the bronze.
"Doping cases are things that happen in Olympic Games," said International Olympic Committee (IOC) president Jacques Rogge.
"We have to tackle that and we are doing that with great energy."
The International Biathlon Union (IBU) could hand Pyleva a two-year ban from the sport, the minimum term under World Anti-Doping Agency (Wada) guidelines.
Under Italy's strict anti-doping laws, Pyleva could even face a prison sentence of between three months and three years, though custodial terms of less than two years are likely to be waived.
Alexander Tikhonov, the head of Russia's biathlon federation, said Pyleva's positive test was the result of medication for a foot injury given by an unauthorised doctor in Russia.
"I don't know whether their defence is credible or not, but it doesn't matter"
Tikhonov said all the Russian athletes had been warned of the dangers of using unauthorised doctors.
"We warned them a thousand times and again - take only medical formulas that are in the team and come only to our doctors," said Tikhonov.
"I have no idea where that doctor who treated Pyleva's foot injury came from."
Tikhonov's theory was backed by Nikolai Durmanov, the head of the Russian Anti-Doping Committee.
"This was 100% the physician's mistake," he said.
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But Wada president Dick Pound said the reasons behind Pyleva's positive test were irrelevant.
"It's a bad thing that somebody is testing positive, but it's a good thing we got her," said Pound.
"I don't know whether their defence is credible or not, but it doesn't matter.
"It's the stuff in your system that counts. That's all that matters as far as the Olympic result is concerned."
Event guide: Biathlon
Pyleva won gold at the 2002 Salt Lake City Olympics in the pursuit and was world champion individual biathlete in 2004 with relay world championship golds in 2000, 2001 and 2005.