Some of these athletes have been openly criticized by their fellow competitors in Rio. In the swimming competition, Yulia Efimova of Russia and Sun Yang of China were the targets of pointed comments.
Efimova, 24, won silver medals in the 100-meter and 200-meter breaststroke events in Rio.
She has served a 16-month suspension for doping, and she tested positive again this year, for the newly banned substance meldonium.
Initially, Efimova had been barred from Rio, along with six other Russian swimmers who either had positive tests on their records or had been named in an investigation of a massive state-sanctioned doping scheme. But a last-minute ruling allowed them to compete.
Efimova was booed every time she raced in Rio and was the object of public derision from the American swimmer Lilly King.
Sun, 24, was called a drug cheat by the Australian swimmer Mack Horton before the mens 400-meter freestyle. Horton, 20, won the gold medal, and Sun placed second. Horton called it a win for the good guys. Sun later won a gold medal in the 200-meter freestyle.
