The run-up to the Games was dominated by Russia's doping scandal, while American rivals Justin Gatlin and LaShawn Merritt have served drug bans.
"People should have faith," said Bolt. "We're weeding out the bad ones. We're going in the right direction."
But the Jamaican added "nothing is guaranteed" when asked whether the sprint races at the Games, which start on Saturday, would be drug-free.
Bolt, who said in February he would retire from athletics after the 2017 World Championships, confirmed this would be his last Olympics.
The sprinter, who holds the 100m and 200m world records, also said he was aiming to become the first man to run under 19 seconds in the 200m.
He could leave Rio with a third successive Olympic treble after winning the 100m, 200m and 4x100m relay titles in 2008 and 2012.
"The 100m is never stressful," Bolt said. "For some reason, when I get to the 200m, I'm always a little bit nervous."
