Brazilian Cesar Cielo won a gold medal at the World Swimming Championships today and in doing so set a dangerous precedent for sport.

His win in the 50-meters butterfly came less than a week after he was cleared of doping, even though the pharmacy he blamed for introducing a banned diuretic into his system denied it was responsible.

The Court of Arbitration for Sport did not hear any evidence presented by the pharmacy in Cielo’s hometown of Santa Barabara d’Oeste and cleared him to compete in this week’s event. (See their decision here.)

Cielo and three other Brazilian swimmers tested positive for furosemide, a banned diuretic, at a competition in Rio de Janeiro in May. They claimed the diuretic got into their system because the pharmacy made an error with their caffeine supplement.

The Brazilian Swimming Federation let the swimmers off with just a warning, even though one had been caught for doping before. (He was banned after the CAS hearing, begging the question if one was found guilty then why not all four.)

That was shocking but not hugely surprising. Cielo is an idol in Brazil and the country’s most successful swimmer. Brazil doesn’t want to see its name sullied or its best medal hope disqualified.

Swimming’s governing body FINA, however, disagreed with the Brazilian federation’s ruling and sought to take a harder line. It appealed to the CAS to ban the swimmers for three months, a relatively short time, but enough for them to miss the World Championships.

Disgracefully, the CAS in Lausanne ratified the Brazilian federation’s decision without a full inquiry. Other swimmers were appalled by the decision and Cielo was booed at the pool today.

That decision looks like a whitewash and can only serve to weaken the struggle against doping in sport.

The positive test may have been an error. But the only way to rid sport of cheats is to punish those caught. At the very least, it forces serious athletes to make 100 % sure they are following all the rules and competing clean.