US Olympic gold medal-winning 19-year-old gymnast Simone Biles has confirmed she has been using medicine to treat attention deficit/hyperactivity disorder (ADHD).

Biles’ acknowledgement came after a hacker group named Fancy Bears’ claimed on Tuesday it had got access to the database of World Anti-Doping Agency (WADA) and figured out that "dozens of American athletes had tested positive", and the Rio Olympic medallists "got their licenses for doping". Among those targeted are Biles and world top tennis players Williams sisters, Serena and Venus, reports Xinhua.

"Having ADHD, and taking medicine for it is nothing to be ashamed of, nothing that I’m afraid to let people know," Biles said on Twitter on Tuesday.

She admitted that she has taken ADHD medicine since she was a child, and she still believes in clean sport.

Ron Biles, Biles’ father, said that he was surprised when he got the call on Tuesday morning from the US Anti-Doping agency.

She has done nothing wrong, her medication was approved and it is a shame someone targeted her medical records, he said.

Steve Penny, President of USA Gymnastics, said on Tuesday that Biles filed the appropriate Therapeutic Use Exemptions (TUEs) paperwork required by the US Anti-Doping Agency and WADA, "and there is no violation".

"Therapeutic Use Exemptions" allows athletes to use substances that are banned unless there is a verified medical need.

The World Anti-Doping Agency (WADA) said on Tuesday that the attack was carried out by a "Russian cyber espionage group" called Fancy Bears, and it was conducted "illegally", but Russia denied on Tuesday any involvement in the recent hacker attacks on its database.

Biles was born on March 14, 1997, in Columbus, Ohio, but she is now living in Spring, a city in northern Houston, Texas.

Having won a combined total of nineteen Olympic and World Championship medals, Biles is the most decorated American gymnast.

Biles won four golds and bronze at the recently concluded 2016 Summer Olympics in Rio de Janeiro.