Sprinter Ato Boldon has told BBC Radio Five Live he cannot be certain the Olympic men's 100m final will be clean of drug cheats.
"I can't say it will 100% clean. There might be somebody there who thinks they might be able to get away with it."
The Trinidad athlete, who has won four Olympic medals, is set to compete in the 100m event.
Boldon said there were only three athletes who could realistically win gold in the blue riband track event.
"I think it'll be a race between Maurice Greene, Asafa Powell and Shawn Crawford, with everybody else in another category unless somebody shows something special in the heats."
The 30-year-old also had a word of encouragement for British sprint hope Darren Campbell.
Campbell was recently angered by comments from former athletics team-mate Colin Jackson, who said it was unlikely a Briton would make the 100m Olympic final.
"I would never count out Darren, although it might be a tough order for him in Athens. He was fortunate that 10.10 seconds got him to the World Championship final last year," added Boldon.
"He might run 10.10 in the Olympic semis and not get to the final.
"But I believe if someone's experienced and fit enough to survive four rounds then there's a chance.
"Darren's experienced and has been there before. I would not be surprised to see him do well and prove Colin wrong."
