Michael Phelps, the most decorated Olympian of all time, believes he never raced against a completely clean international field.
The American former swimmer, 31, wants US lawmakers to push for global anti-doping reforms.
"I don't believe I've stood up at an international competition and the rest of the field has been clean," he said.
Phelps was giving evidence at a US House of Representatives hearing into improving anti-doping measures.
The US government helps to fund the World Anti-Doping Agency (Wada) and the committee could recommend giving more money to the organisation.
Phelps, who won 23 Olympic gold medals, added: "Throughout my career, I have thought that some athletes were cheating and in some cases those suspicions were confirmed.
"Given all the testing I and others have been through, I have a hard time understanding this."
Phelps spoke out against drugs cheats at the Rio Olympics.
When asked about Russian Yulia Efimova, who won Olympic silver after two positive tests, he said dopers returning to elite action "breaks his heart".Rio Olympics 2016: Yulia Efimova says 'Rio was awful - it was war'
A report commissioned by Wada and written by lawyer Richard McLaren claimed in December that more than 1,000 Russian athletes benefited from a state-sponsored doping programme between 2011 and 2015.
Travis Tygart, the chief executive of the US Anti-Doping Agency, also addressed Tuesday's hearing at the House of Representatives. He told politicians the International Olympic Committee had let down clean athletes after opting against a blanket ban of Russian athletes at Rio 2016.
The IOC instead allowed individual sports to decide their own policy.
"At least two Olympic Games were corrupted and, at the Rio Games this past August, scores of Russian athletes were allowed to compete without credible anti-doping measures," said Tygart.
"When the moment came, despite mountains of evidence and vocal opposition from anti-doping leaders and clean athletes from around the world, the IOC chose to welcome the Russian Olympic Committee to Rio."
Meanwhile, Russian President Vladimir Putin has denied government officials were complicit in doping the country's athletes.
"In Russia we never had, don't have, and I hope won't have a state-sponsored doping programme. On the contrary, there will only be a fight against doping," he said in a television broadcast.
He added a new doping control system was being put together.