MOSCOW, September 13. /TASS/. Banned performance enhancing substances, which the World Anti-Doping Agency (WADA) allegedly allowed US tennis stars Serena and Venus Williams for legally taking, are usually prescribed for terminally-ill patients, a Russian anti-doping expert told TASS on Tuesday.

Fancybear.net web portal of hackers announced earlier that it hacked the ADAMS and leaked documents proving that WADA allowed US tennis stars Serena and Venus Williams as well as US four-time Olympic Champion in gymnastics Simone Biles and women’s basketball player Elena Dolle Donne taking banned performance enhancing drugs.

"We are talking here about very strong substances, in other words we are practically talking about narcotics, and such substances are extremely unusual for sports and their possession is illegal in many countries, including in Russia, and is fraught with considerable prison terms," Professor Nikolai Durmanov said in an interview with TASS.

According to the information posted on the hackers’ Fancybear.net website, "Serena Williams, world's top tennis player, is taking oxycodone and hydromorphone (opioids), prednisone, prednisolone, and methylprednisolone as well."

"Her sister Venus Williams is used to take prednisone, prednisolone, triamcinolone and formoterol," according to Fancybear.net web portal.

Durmanov said that the use of such drugs "is extremely unusual for sports since medications of such effect are usually prescribed for terminally-ill patients, while we are talking here about Olympic champions."

"My general question is - how sick must be an athlete to be prescribed such strong drugs? Perhaps he or she should have been rather hospitalized instead of going to the Olympic Games?" the expert said.

Commenting on the allegations, the International Tennis Federation (ITF) said in its statement that "If you have a medical condition that requires Use of a medication or other product containing a substance or method that is on the 2016 Prohibited List, a Therapeutic Use Exemption (TUE) is required."

"A TUE permits you to use the prohibited medication or method without committing an Anti-Doping Rule Violation, providing that all such use is in accordance with the conditions of the TUE," the statement from the ITF said. "Both (Serena and Venus Williams) were granted a TUE."

WADA announced in its official statement on Tuesday night that Russia was behind the attack on the ADAMS database, but did not specify whether the leaked information was true.