The suspension is effective as of February 5, and will apply to all football activities at both national and international level.
The substance furosemide was discovered in the Cameroon international’s urine in an out of competition test on 31 October last year, but Ajax claim Onana took it “unwittingly” after feeling unwell.
“On the morning of 30 October Onana did not feel well. He wanted to take a tablet. Unwittingly, however, he took Lasimac, a drug that his wife had previously been prescribed,” an Ajax statement said.
Ajax said they would appeal the ruling at the Court of Arbitration for Sport (CAS).
The club added: “Onana made a mistake with the product and mistakenly took his wife’s medicine, which ultimately led to UEFA’s action against the goalkeeper. The football association’s disciplinary body ruled that he did not intend to cheat. However, the European Football Association believes that, according to the prevailing anti-doping rules, an athlete has a duty at all times to ensure that no prohibited substances enter the body.”
Onana and Ajax will appeal the ruling at the Court of Arbitration for Sport (CAS).
Ajax’s managing director Edwin van der Sar said: “We explicitly renounce performance-enhancing drugs; we 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 many years and is very popular with the fans. We had hoped for a conditional suspension or a much shorter one than this twelve months, because it was demonstrably not meant to strengthen his body and thus improve his performance.”