Dinamo Zagreb midfielder Arijan Ademi failed a drug test after their Champions League match against Arsenal last month and is under investigation by UEFA, the club said on Wednesday.
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"We have been informed about the results of the drug test and that our player Arijan Ademi tested positive," the Croatian title holders said on their website.
"UEFA has opened a proceeding against him and in compliance with the football governing body's rules, Dinamo cannot comment until the disciplinary procedure is over and the B sample result is announced."
Dinamo beat Arsenal 2-1 at home in their Group F opener on September 16 before losing 5-0 at Bayern Munich two weeks later.
They are third in the section with three points from two games. Bayern lead the standings with a maximum six points while Olympiakos Piraeus are ahead of Dinamo on goal difference.
Dinamo released a statement to say club officials were "surprised" with Ademi's failed drugs test.
"He had been tested six times in the past year and the results always came back negative."