The IOC said it was unhappy that it had not been informed of four positive tests on the cyclist, who was also caught taking banned substances during the Tour.
IOC medical officials did not name the rider but said there had been only one doping case during the Tour. Uzbekistan's Djamolidine Abdoujaparov, a former points winner, was thrown out of the race after failing a test. IOC medical officials said the unnamed rider had produced positive tests four times before the Tour for the banned substances clenbuterol and bromantan. At the time, Abdoujaparov said he had been confused by the positive test.
"I don't know what happened," he said. "In eight years as a professional, it never happened to me. I don't understand." He added that he was probably given a banned product by a member of his Lotto team as a painkiller following a crash in the first stage.
"We have to put the house in order in this matter," the head of the IOC's medical committee, Prince Alexandre de Merode, said. "We are going to investigate."
The cases were reported to cycling federations and French sporting authorities but not to the IOC, which is supposed to be informed of all positive tests.
Bromantan, a stimulant-cum-masking agent, and the cattle-fattening compound clenbuterol have controversial histories. Seven competitors tested positive for bromantan at last year's Atlanta Games but were cleared of any punishment after an IOC hearing concluded there was not enough evidence to prove it was a stimulant. The substance was put on the banned list from 1 February, however.
Clenbuterol was the drug at the centre of an athletics scandal in 1992, when Germany's former double world sprint champion, Katrin Krabbe, was banned after testing positive.
It is a drug used to help asthma sufferers and helps athletes to breathe more easily. But there are also arguments that it has anabolic properties and boosts muscle development.
In the past it has not been licensed for human use, but can be given to horses and cattle. The IOC banned it in 1992 when two British weightlifters were withdrawn from the Barcelona Olympics for using it.
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