"We have received a report from the games organisers that Kodei has lost his silver medal after he failed a drugs test in Manchester," said Tony Nezianya, a spokesman for the Nigeria Olympic Committee (NOC).
"We have asked the wrestling federation to investigate this case and we are still waiting for their findings."
Kodei was beaten in the final of the 96kg class at the Manchester games by Dean Schmeichel of Canada.
India's Anil Kumar, who won a bronze in that weight class, will now be awarded the silver medal.
Kodei, who has represented Nigeria for over a decade, becomes the second Nigerian wrestler to fail a doping test recently.
Friday Agumnowie was sent home before the games in Manchester after failing a test.
Officials said that the highly-rated Kodei faces a two-year ban should his B-sample prove positive.