Chryste Gaines, Sandra Glover, Eric Thomas and Chris Phillips have all been warned by the US Anti-Doping Agency after testing positive for modafinil.
Gaines tested positive for modafinil at last year's US championships
But the American athletes have not been barred from competing in the Athens Olympics and other events this year.
Sprinter Gaines and hurdlers Glover and Thomas failed tests for the stimulant at the US championships last June.
Phillips tested positive at the world championships last August, where he finished fifth in the 110m hurdles.
All four athletes will lose all results from the competition in which they tested positive.
Thomas won the US 400m hurdles title, Glover was third in the 400m hurdles and Gaines was fifth in the 100m.
Gaines competed at the 1996 and 2000 Olympics and Glover and Thomas were members of the US team at the 2000 Sydney Games.
Modafinil was banned because it is related to substances banned by athletics' world governing body the IAAF.
At the time it was considered a minor stimulant that warranted only a disqualification.
This year, however, it has been classified as a serious stimulant on the World Anti-Doping Agency banned list and carries a two-year ban.
Sprint champion Kelli White is still contesting her positive tests for modafinil at the US and world championships. She expects an arbitration hearing next month.
White, who could lose her world titles in the 100m and 200m, said she was prescribed modafinil for a sleeping disorder.
White and Gaines are coached by Remi Korchemny, one of four men charged in February with distributing performance-enhancing drugs.
Phillips said Korchemny gave him a modafinil pill at the world championships to help him overcome jet lag.
Korchemny and his three co-defendants have protested their innocence.