Phelps, who is aiming to break Mark Spitz's 1972 record of seven golds at a single Games, won the men's 200 meters individual medley on Friday and has now won every event he has swum at the Water Cube and broken the world record each time.
- Phelps, the billion dollar man?
Asked at a media conference how, given recent doping cases involving several US athletes he would respond to suggestions his achievements are too good to be true, Phelps was emphatic.
"Anyone is able to say whatever they want. I know that I am clean. I did 'Project Believe' with USADA (US Anti Doping Agency) where I purposely wanted to do more tests to prove that," said Phelps. "People can question all they want but the facts are facts and I have the results to prove it."
'Project Believe' is a voluntary testing program that also includes sprinter Allyson Felix and decathlete Brian Clay where the participants provide additional blood and urine samples beyond the regular testing regime.
Phelps's image features prominently on USADA's website and he is also featured in the organization's 'My Victory' campaign, which encourages young athletes to take a pledge against doping.
The American, who now has an unprecedented 12 career Olympic golds, competes across the disciplines and said that meant he needed a specialized and extensive training program.
"With a program like I have, I have to work on speed and endurance and all four strokes. I need the speed to take it out (at the start) and the endurance to carry it through," he said.
"You can't do it on talent alone. A lot of hard work, a lot of dedication.
"It has been a combination of everything...trying to sleep as much as I can, trying to recover, set up every workout as well as I can and just try to bank a lot of workouts back to back."