Usain Bolt admits he cannot be completely sure that his sprint rivals at the Rio Olympics are all clean.
The Jamaican is aiming to complete the 'triple triple' of Olympic sprint titles in Brazil by retaining his 100, 200, and 4x100 metres relay titles.
The build-up to the Games has been dominated by the Russian doping scandal, while the world number ones over 100m and 200m - Americans Justin Gatlin and LaShawn Merritt - have both served drug bans.
"In life nothing is guaranteed," the 29-year-old said when asked if he could be certain the sprint races in Rio would be drug-free.
"For me going out there I never worry about that. I just go out there and compete.
"I think we're going in the right direction, I must say. I think we're weeding out the bad ones. We have to go through a rough time before we get to the good times."
