The substance furosemide was discovered in the Cameroon international’s urine in an out of competition test on 31 October last year.
The Dutch giants note that the disciplinary body acknowledged that the player had “no intention of cheating”, while managing director Edwin van der Sar has argued the substance was not performance-enhancing.
The club explains that the player was feeling unwell the day before the test took place and took a pill known as Lasimac, prescribed for his wife, to ease the pain.
The club then adds that Onana “mistakenly took his wife’s medicine”, and that the ban was enforced with the disciplinary body maintaining that there is “a duty at all times to ensure that no banned substances enter the body”.
The ban is effective from today and “applies to all football activities, both national and international”, which would see Onana miss Ajax’s Europa League Round of 32 tie with Lille later this month.
The club confirmed their intention to appeal the decision at the Court of Arbitration for Sport (CAS).
Van der Sar: “We explicitly renounce performance-enhancing drugs, we obviously stand for a clean sport. This is a terrible setback, for Andre himself but certainly also for us as a club.
“Andre is a top goalkeeper, who has proven his worth for Ajax for years and is very popular with the fans. We had hoped for a conditional suspension or for a suspension much shorter than these twelve months, because it was arguably not intended to strengthen his body and thus improve his performance.”