The International Olympic Committee spokeswoman Michele Verdier revealed that senior IOC officials "unanimously" supported a recommendation by the medical commission to issue a strong reprimand to the athlete and the Irish team and not to enforce a ban.
The 21-year-old runner admitted taking two Robitussin decongestant pills to clear a "stuffy nose" without telling doctors. The Irish chief team doctor, Joe Cummiskey, had protested to the medical commission that the athlete had "made a terrible mistake" and had not used the drug to boost her performance.
Ms Verdier said that because of competition between the Irish Olympic officials and athletics bosses, their athletes were "not getting the right instructions" about which drugs were outlawed.
An Irish Olympic spokesman, David Guiney, said: "We are naturally absolutely delighted with the outcome of this very unfortunate incident. It was a complete accident and Marie is devastated, but fortunately the authorities have acted realistically and properly and we will be very mindful of what they have said."
Verdier explained that Robitussin contains phenylpropanolamine, which is, oddly, both on the IOC list of banned and legal substances. She said the Irish team had not made it clear to the athletes which category the drug she had taken came into, therefore they decided not to punish the athlete with a ban but to opt for the strong reprimand. MacMahon finished 14th in her heat of the women's 5,000 metres on Friday.
Verdier also announced that the Russian sprinter Marina Trandenkova has been disqualified from the Games after failing a drug test. Trandenkova, who came fifth in the women's 100m final on Saturday, is the fourth Russian to test positive for drugs, all for using bromantan, a stimulant-cum-masking agent.
The Russians have appealed against the disqualifications, arguing that bromantan is not specifically cited on the IOC's list of banned substances. But Verdier said: "It is the role of the IOC to detect banned substances. This substance was declared illegal on 5 June."
A leading IOC official claimed that the drug had been used for many years but scientists could not detect it until recently.
Prince Alexandre de Merode, chairman of the IOC's medical commission, said traces were detected in as many as 20 tests in the past two years, all involving athletes from the former Soviet Union.
"It's made by the Russian army for army troops," de Merode said. "I am told it is available on the black market in Russia, including on the streets in Moscow."Reuse content