SYDNEY, Australia -- Australian swimmer Richard Upton, a relay silver medalist at this year's world championships, has returned a positive test to a banned masking agent.
Upton said he was told last week that he tested positive to probenecid, which he said he was given by his doctor to help fight a chest infection.
"I don't know how to take this, it's come as a shock," Upton said. "It's hard for me to know what to do because I feel I have done nothing wrong."
Upton is a member of Sydney's Carlile swim team -- headed by Forbes Carlile, a member of the World Coaches Association's anti-doping committee and a leading critic of Chinese swimmers for their alleged drug use.
Upton said he had received treatment for the infection on Feb. 14 and his doctor had given him a penicillin injection and two tablets of probenecid.
Probenecid is considered a masking agent on the International Olympic Committee's banned substance list because it prevents other drugs being excreted in urine. It would be used to help him retain the penicillin.
He failed an out-of-competition test on Feb. 17 and left Australia the next day to compete at the World Cup meets in Hong Kong and Beijing. Upton placed second to Russian world champion Alexander Popov at both meets.
Upton said he had a letter from his doctor explaining the use of the masking agent and he has support from Carlile.
"I think it would be impossible to apportion any blame to him under these circumstances -- he was a very sick boy at the time," Carlile said.