BEIJING - China's controversial Olympic swimming star Sun Yang failed a doping test in May and was subsequently banned for three months, the official Xinhua news agency reported Monday.
The ban, following a positive test for the stimulant trimetazidine, was imposed in July, the agency said, citing the China Anti-Doping Agency (CHINADA).
Sun competed in the Asian Games in September, where he took gold in the 1,500 and 400 metres freestyle, and the 4 x 100 relay.
Xinhua did not immediately explain why the positive result had only come to light now, or how Sun was able to take part in the Incheon event.
It said that the 22-year-old tested positive on May 17 during China's national swimming championships, waived his right to have his B sample tested, and defended himself in a hearing in July "where the experts decided to hand him a three-month ban".
Trimetazidine was added to the World Anti-Doping Agency's banned list in January this year, Xinhua said. Sun said he used it for medical reasons and had been unaware that it was included on the list, it added.
Sun won the 400 and 1500 metres freestyle events at the 2012 London Olympics, and also has five world championship golds to his name, but has frequently faced controversy during his career.
At the Asian Games he called the Japanese national anthem "ugly", a comment for which he later apologised.
It was only the latest incident to embroil Sun, who has frequently battled with authority, leading to jail time and prolonged suspensions from the swimming pool.
In 2013, he was suspended from commercial activities and warned about his personal behaviour after a battle with his coach over a relationship with an airline stewardess. Sun had missed training sessions to go on dates.
The swimming star's notoriety grew in November last year when he was caught driving a relative's Porsche without a licence following a collision with a bus. He was jailed for one week and suspended from swimming for six months.
After the ban was lifted, Sun returned to the pool at the national championships, winning the 200m freestyle title days before his positive test.