Belfast boxer Brian Magee is serving a six-month ban for use of the prohibited stimulant Oxilofrine, the UK Anti-Doping Agency has revealed.
The 38-year-old Magee tested positive after his defeat by Mikkel Kessler in a WBA super-middleweight title challenge in Denmark in December.
He was informed of his ban in February, and it ends on 29 July.
Magee said he thought someone was joking when told about the test result and feared it could end his career.
It is believed Magee escaped a two-year suspension by showing the substance had been an unlabelled ingredient in an 'over the counter' sports drink.
"I was in total shock when I was told I had tested positive, and I honestly thought someone was playing a joke," said Magee.
"All I could think was I could get a lifetime ban or even a two-year ban which at my age would have meant my career was as good as over.
"I have had 30 or 40 tests in my career and never tested positive. I have always been extremely careful about everything I have taken, and I didn't do anything differently before the Kessler fight."
Magee said he consumed the banned substance via a contaminated over-the-counter supplement he had been using to combat a cold. The boxer is understood to be considering legal action against the supplier.
UK Anti-Doping accepted Magee's version of events, saying it was satisfied he had not sought to enhance his performance.
The boxer, who fought Carl Froch in 2006 and is a former British and European champion, said he was intent on returning to the ring.
"I have already got back in the gym and I can't wait to fight again," he added.
"I have not had any bad beatings and I think there are still a number of big fights out there for me."
Jamaican sprinters Asafa Powell and Sherone Simpson both tested positive for the same stimulant following the Jamaican national trials last month.