(CNN) -- He's the fastest runner ever, but Usain Bolt accepts that it will take people time to believe that he has no illegal assistance.
The Jamaican has blitzed his rivals in the past two years, winning the 100 and 200 meters at the 2008 Olympics and this year's World Championships, setting new best marks every time.
He first set the 100m record in only his fifth attempt in June last year, and shaved an incredible 0.11 seconds off his own mark when winning the world title in a time of 9.58 in Berlin in August.
But while five of his Jamaican teammates tested positive for the stimulant methylxanthine before the event in Germany, the 23-year-old has maintained a clean image.
In April he admitted trying marijuana as a child, but Bolt insisted that he has never taken drugs while a competitive athlete.
"I'll continue running fast, I am clean without a doubt," he told reporters at a charity event in London on Thursday.
"As soon as you start running fast there's going to be a problem. It's going to take a couple of years for people to start believing that you're actually clean.
"If you continue running fast and doing it clean, then over a period of time all the tainting will go away. People will continue to say it until that period of time.
"The sport has been through a lot, but it's changing, definitely. The IAAF is doing a very good job of cleaning up the sport."
Bolt is confident that he can maintain his meteoric rise and defend his Olympic titles when the British capital hosts the Games in 2012.
"The Olympics is going to be a very big thing for me because I did extraordinary things in Beijing," he said.
"I'm definitely coming here to defend my titles and I'm looking forward to it because there are a lot of Jamaicans here so I know it's going to be a good Games. I'm just going to come here and do my best as always."
He was speaking at the launch of The Zeitz Foundation and The Long Run -- a new global initiative by Jochen Zeitz, chairman and chief executive of sportswear maker Puma.