Olympic and world champion swimmer Sun Yang quietly served a three-month doping ban last summer, Chinese officials announced on Monday.
The country's antidoping agency said the ban was not disclosed earlier as a matter of policy.
"This is huge bad news but we will not cover it up," the agency's deputy director told the official Xinhua news agency. "We announce positive cases and test statistics in our quarterly reports just as WADA requires."
Sun, who won gold in the 400- and 1,500-meter freestyle events at the 2012 London Olympics, tested positive for the stimulant Trimetazidine at the Chinese national championships last May.
The swimmer said he was taking medication for a heart condition and was not aware it was prohibited.
"I have been tested numerous times during years of training and competition and I had never failed once before," Sun was quoted as saying. "I was shocked and depressed at that time, but at the same time it made me cherish my sporting life even more. I will take it as a lesson and be more careful in the future."
His doctor, Ba Zhen, was banned for a year, the international swimming federation announced.