It marked the 37-year-old's second doping controversy of the year after a failure for meldonium in April.
Povetkin, an Olympic champion at Athens 2004, had been due to fight Canadian Bermane Stiverne for the vacant interim WBC heavyweight title yesterday.
But the WBC withdrew their recognition of the fight and launched an investigation following the failure.
The Russian instead fought France’s Johann Duhaupas in a non-title fight in Yekaterinburg.
"What sense does it make to use doping and fail the last test?" Povetkin told the TASS news agency after knocking-out his opponent in the sixth round.
"It is all senseless.
"I don’t know how the substance got into my body.
"I don’t even have any assumptions.
"When I learnt about it I was getting uneasy, I took it so hard."