Olympic great Michael Phelps believes he has never competed in an international race completely free of doping cheats.
The American, a 23-time Games gold medallist, was speaking at a congressional subcommittee hearing on anti-doping on Tuesday.
Phelps, 31, said he felt every international race he had been involved in had at least one cheat competing.
"I don't believe I've stood up at an international competition and the rest of the field has been clean," he said.
"I don't think I've ever felt that and I know when I do stand up in the US, I know we're all clean because we go through the same thing."
Phelps, who won five gold medals at Rio 2016, said he was frustrated by doping cheats, calling for a more unified testing process.
"Throughout my career I have thought that some athletes were cheating and in some cases those suspicions were confirmed," he said.
"Given all the testing I and others have been through, I have a hard time understanding this.
"In addition to the tests in the competitions, I had to notify USADA [United States Anti-Doping Agency] where I was every day so they would be able to conduct random tests outside of competition. This whole process takes a toll but it's absolutely worth it to keep the sport free and fair.
"I can't describe how frustrating it is to see athletes break through performance barriers in unrealistic time frames knowing what I had to do to go through that. I watched how this affected my team-mates as well. Even the suspicion of doping is disillusioning for clean athletes.
"To believe in yourself through sport, you need to be able to believe in the system that safeguards clean sport and fair play.
"All athletes need to be held to the same standards which need to be implemented and enforced with consistency and independence."