Swimmer Michael Phelps believes "something needs to change" after lamenting the lack of "clean sport".
Phelps, who will compete in his fifth Olympics when the Games start next week, has voiced his opinion over the doping issues that continue to cast a shadow over the build up to the showpiece event in Rio.
"We all want clean sports and everyone on the same playing field," he said.
"I don't think I've ever competed in a clean sport – but there's not much other than me I can control."
The 31-year-old has 22 Olympic medals in his trophy cabinet and will be looking to add to his 18 golds when the games begin on Saturday.
"Something needs to change with all sports, not just swimming," he added.
"As athletes you want to compete on a level playing field – and how many Olympics have we had and been saying this at now? It's sad someone at the top can't get a handle on this."
The American was chosen by his fellow team members to carry the USA flag at Friday's opening ceremony.
"I'm honoured to be chosen, proud to represent the US, and humbled by the significance of carrying the flag and all it stands for," he said.