Rio 2016: Michael Phelps wants action to be taken against dopers
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Michael Phelps says it "breaks my heart" that convicted dopers are allowed to compete at Rio 2016.
The 31-year-old American has come out of retirement to compete in his fifth Games, and has already picked up a record 19th gold medal.
He has spoken out against drugs cheats after twice-banned Russian Yulia Efimova won silver in the women's 100m breaststroke on Tuesday.
"It breaks what sport is meant to be," said Phelps.
"It's sad that today in sports in general, not just only swimming, there are people who are testing positive who are allowed back in the sport - and multiple times.
"I believe sport should be clean and sport should be on an even playing field, and I think that it's sad that in sports today we have people who are testing positive not only once but twice and still having the opportunity to swim at this Games.
"It breaks my heart and I wish somebody would do something about it."
However, R-Sport agency Russia's sports minister Vitaly Mutko defended his compatriot.
"Yulia is without doubt a good girl," he said.
"After undergoing such a terrible ordeal she has shown character, willpower and courage, even though - as we can see - certain provocations are still happening."
Mutko pointed to competitors from other countries who have been allowed to compete despite doping bans.
"It's surprising that supporters from certain countries, who have five or 10 dopers that had served bans for much more serious doping violations in their line-ups, do not boo their athletes," he said.
Efimova was banned for 16 months in 2013 after traces of an anabolic steroid were found in her system.
Earlier this year she was given a provisional suspension after testing positive for meldonium, but the International Swimming Federation lifted the ban after advice from the World Anti-Doping Agency.
The International Olympic Committee had ruled any Russian athlete who had been sanctioned for doping would be banned from competing in Rio, but Efimova was allowed to take part following an appeal to the Court of Arbitration for Sport.
Phelps' compatriot Lilly King beat Efimova in the 100m breaststroke and has been outspoken in her disdain for the Russian.
And Phelps backed King's attitude, saying: "I think people should be speaking out more. You know I think (Lily) is right. I think something needs to be done."
Efimova, who has been booed at every race so far at Rio, said: "It's very sad and upsetting. I know a lot of clean athletes, banned just because of Russia.
"All the stuff that happened with me was unbelievable. I made mistakes and I was banned for six months. The second time was not my mistake."
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