The British sprinter was banned for 22 months after testing positive for two banned substances.
The AIU and WADA found him not guilty of taking banned drugs.
The men's 4x 100m silver medal was taken away from Britain after Ujah tested positive for two banned substances.
The ban will last for five years, ending on 5 June 2023.
The sprinter's anti-doping rule violation was found to be a result of his ingestion of a contaminated supplement.
The period of ineligibility was reduced due to the prompt admittance of the violation by the 28-year-old.
"After a thorough examination of the facts, we were satisfied that Mr Ujah did ingest a contaminated supplement, but he was unable to demonstrate that he was entitled to any reduction in the applicable period of ineligibility based on his level of fault."
Athletes taking supplements are at risk as they can be adulterated with banned substances.
Athletes owe it to their competitors to be sure before putting anything into their body. Leave it out if there's any doubts.
At the time of the positive test, Ujah said he had "unknowingly consumed a contaminated substance" and he would "regret for the rest of my life".
Zharnel Hughes, Richard Kilty, and Nethaneel Mitchell-Blake were stripped of a medal after being part of the 4x 100m relay team that was disqualified.
He didn't challenge the decision made by the anti-doping division of thecas
According to the UK Anti-Doping website, S-23 is a drug designed to have the same effects as Testosterone.
The AIU and WADA have given the go-ahead for Ujah to return to competition in time for the World Championships in Hungary.
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