Jepchirchir won women's marathon gold at the Tokyo Olympics in 2021 and the world title last year. The 33-year-old Kenyan runner is also a former world record holder in the half marathon.
The AIU, which investigates doping in track and field on behalf of the sport's governing body World Athletics, posted on X that "an out-of-competition sample, collected in Kenya prior to her recent Sydney Marathon victory, returned an adverse analytical finding for EPO."
Jepchirchir won the Sydney Marathon in August. Her representative Gianni Demadonna declined to comment when contacted by The Associated Press.
In most doping cases in international sports, athletes are provisionally suspended from competing until the case is either resolved with a verdict or dropped.
Jepchirchir's win in Sydney was another award in a glittering career which includes victories in the 2021 New York Marathon, the 2022 Boston Marathon and 2024 London Marathon. She is ranked fifth in the world for the women's marathon.
Jepchirchir also made history when she broke the half marathon world record in 2017 while in the early stages of pregnancy, according to governing body World Athletics. She gave birth to a daughter eight months after that.
Sydney Marathon organizers said in a statement Friday they were "extremely disappointed" to hear of Jepchirchir's suspension.
"Jepchirchir is entitled to due process, and we will make no further comment on the case until that process has concluded," the statement said. "Our commitment to clean sport is unwavering. Every athlete who lines up in Sydney deserves to race on a level playing field, and every participant and spectator deserves to trust the result they see at the finish line."
It's the latest in a long series of doping cases involving distance runners from Kenya, which was awarded the 2029 world track championships last month. It will be the first African country to host the event, but Kenya has long faced scrutiny over performance-enhancing drug use by its world-leading runners.
Women's marathon world record holder Ruth Chepng'etich, who is also from Kenya, was banned for three years in 2025 after admitting a charge related to a positive test for a banned diuretic. Former New York Marathon winner Albert Korir was given a five-year ban in March.
World Anti-Doping Agency president Witold Banka said this month Kenya needs a "more determined approach" to retain its credibility in sports.
"Kenya proudly calls itself the home of champions. But for that home to remain respected and credible, it must also be clean," Banka said during a visit to the East African nation. "That is why Kenya needs a strong culture of clean sport, zero tolerance for doping, and a more determined approach to preventing and fighting doping."
The suspension of Jepchirchir was announced a day after the AIU said it issued a six-year ban to Ruti Aga, a top marathon runner from Ethiopia. The AIU said Aga tested positive for a banned substance and then submitted a fake prescription for the substance from a clinic which turned out not to exist.