The Australian Ben Tune has been cleared to play by the International Rugby Board's (IRB) Anti-Doping Advisory Committee, which ordered a review of his case when the Australian Rugby Union (ARU) decided not to suspend the wing after it emerged that he had taken the banned substance Probenecid.
Tune faced a possible two-year ban which would have kept him out of the World Cup had the IRB decided to go against the ARU's ruling.
However, the IRB have promised a second investigation into the conduct of the ARU and the Queensland Rugby Union who did not inform world rugby's governing body as required until the matter was exposed by an Australian newspaper.
Tune was prescribed Probenecid in March last year in a bid to boost antibiotics he was taking for a knee infection.
His medical advisers, while aware that it was prohibited, mistakenly believed the drug was permitted for therapeutic purposes if this was disclosed in advance to the Australian Sport Drug Agency (ASDA).
Tune then played in two Super 12 games before ASDA contacted the ARU to tell them that he should not be playing.
He was then stood down for four matches and his urine was tested by the Reds' medical staff four times. He was not allowed to play until the testing showed no trace of the drug.
However, IRB rules require home unions to report all failed dope tests to the IRB within 14 days. Neither Queensland nor the ARU informed the IRB of his case until August this year when the story was widely reported in the media.
A statement from the IRB said: "Mr Tune remains eligible to continue playing. The procedures followed by the IRB recognise that there are two separate and distinct issues relating to this matter.
"The Anti-Doping issue in relation to the player was, in the interests of fairness, treated as a priority, and has now been concluded.
"The second aspect concerns the other events surrounding the matter. In this regard an enquiry will now be initiated into the background and circumstances of the case."
Newcomer Adam Freier could find himself thrown straight in at the deep end when Australia tour Argentina and Europe later this year.
The coach Eddie Jones has given the biggest hint so far that the hooker might grab a place in the side as early as the opening Test against the Pumas on 2 November.
It had been expected the 27-year-old would be the understudy to regular Jeremy Paul but Jones said: "He has a definite possibility of being in the starting line-up. We will probably look at some changes right from the start."
Jones will use the tour to try out new combinations ahead of next year's World Cup as well as making sure none of his players become complacent.
The full-back Mat Rogers and the wing Wendell Sailor are expected to move up from the bench, with Stirling Mortlock moving from the wing to outside centre. Up front David Lyons should play and Owen Finegan could be switched from back to second row.
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