Olympic and world swimming champion Cesar Cielo has tested positive for the banned substance furosemide but has escaped suspension.
The Brazilian Aquatic Sports Confederation says Cielo, the world record holder in the 50 and 100 metres freestyle and the gold medallist in the 50 freestyle in Beijing, and three other swimmers tested positive for the diuretic at the Brazilian national championships in May and had been warned.
The confederation says it is proven there was no improvement in their performances.
The other three swimmers were named as Nicholas Santos, Henrique Barbosa and Vinicius Waked. The four had their results scratched.
