Tour de France winner Alberto Contador is going to have to come up with a better explanation for why he failed a drug test than tainted meat.
"It's been raised before, it's been heard in a couple of cases and rejected," said WADA director general David Howman. "It's not unusual."
Contador was provisionally suspended by the International Cycling Union (UCI) last month after the positive test results were made public.
Howman said that Contador needed to provide actual evidence of his claim.
"The issue is, can you prove it? It's a pretty hard thing to prove that is where it (the banned substance) comes from," he said.
Contador has another problem in that plastic residues were reportedly found in his blood along with the supplement, suggesting that he may have undergone blood transfusions, which are also banned in the sport.
The three-time Tour winner announced that he will not attend the 2011 race's presentation in Paris next week due to his suspension.