Korir, 32, tested positive for the synthetic form of erythropoietin (EPO) that stimulates red blood cell production during out-of-competition tests in Kenya in October 2025.
The AIU said that Korir had received a one-year reduction from the original six-year suspension "based on an early admission and acceptance of the sanction".
His five-year ban will run from January 8th, 2026, the date he was provisionally suspended, until January 7th, 2031.
Korir won the 2021 New York marathon in a time of 2hr 08min 22sec and came third in 2023 with a personal best time of 2:06:57.
He won the Ottawa marathon in 2019 and 2025.
Korir's sanction comes nearly six months after compatriot Ruth Chepngetich, the current world marathon record holder, was banned for three years after admitting the use of Hydrochlorothiazide (HCTZ), a banned diuretic used as a masking agent.
Running is a way out of poverty for many in Kenya, putting pressure on many to turn to drugs, especially since the country lacks the sophisticated infrastructure to train stars.
Kenya worked to clean up its image after a string of doping scandals around the 2016 Rio Olympics led to it being declared non-compliant by the World Anti-Doping Agency (WADA).
More than 140 Kenyan runners, mainly long-distance athletes, have been sanctioned for drugs offences since then.
In June 2024, Kenya handed out its first lifetime ban to marathon runner Beatrice Toroitich and a six-year ban to 10km record-holder Rhonex Kipruto.
