Gil Roberts has been cleared of doping after the Olympic gold medallist was found to have ingested a banned substance through kissing his girlfriend.

Winner of gold in the 4x400metre relay at the Rio Games last year, American sprinter Roberts tested positive for probenecid on March 24.

The 28-year-old pleaded his innocence, telling the US Anti-Doping Agency (USADA) he contaminated the substance from his girlfriend, Alex Salazar – who had been taking medication to treat a sinus infection.

Salazar pulled apart a capsule of medicine that she had obtained in India, poured the contents of the capsule in her mouth, then washed it down with water.

And an arbitrator ruled in Roberts' favour, claiming "this was not a case of intentional doping".

"[F]or Roberts it must have been like lightning out of a clear blue sky for him to learn that by kissing his girlfriend this time that he was exposing himself to a prohibited substance," Judge John Charles Thomas said.

The report mentioned that Roberts had been "tested for nearly a decade and had been found to be clean", while "the amount of probenecid found in his sample is too small to have a masking effect, nor was there any evidence that he had ingested probenecid from any other sources."