The Pan American Sports Association says four athletes have been pulled from the international competition after positive drug tests.
The association says three of the athletes — Cinthya Vanessa Dominguez Lara of Mexico, Nelson Gomez of Puerto Rico and Javier Jesus Ortiz Angulo of Colombia — tested positive for anabolic steroids.
It says the fourth, Mario Mercedes Castillo of the Dominican Republic, tested positive for a banned stimulant.
Eduardo De Rose, chairman of the organization's medical commission, says every major sporting event has a number of anti-doping violations, but it's surprising to see this many cases so early in the Games.
Dominguez Lara is a weightlifter and the other three are baseball players.
The Peruvian delegation says swimmer Mauricio Fiol, the silver medallist in the 200-metre butterfly, has also tested positive for steroids and has been suspended from the team.
In a statement, the Peru team says, "We are profoundly sorry for the incident, which affects not only the athlete but all Peruvians."
Fiol says, "I don't understand how this could have happened. I've always promoted fair play."
Fiol has the option of asking for the result of the B sample.
Canada could benefit if Fiol is stripped of his medal as Zack Chertat's bronze would be upgraded to silver while teammate Alec Page would move up to third place.
The Pan Am Sports Association says it will not confirm or comment on any potential doping cases for 48 hours after receiving test results in order to give the affected athletes a chance to address the allegations.