Dinamo Zagreb midfielder Arijan Ademi has been banned for four years after failing a drug test following a Champions League match against Arsenal in September.
The 24-year-old central midfielder, who has made three international appearances for Macedonia, failed the test after Dinamo's 2-1 home win over the Gunners in their Group F opener.
His B-sample also came back positive but Ademi nonetheless claimed he was innocent.
"I am at a loss of words to describe how terrible I feel," he said. "I am not guilty, I know that I did nothing wrong but still received a drastic punishment."
Dinamo announced the news on their official website and vowed to support the player.
"The UEFA disciplinary committee made the harshest possible decision and suspended Ademi for four years," the statement said.
"We couldn't imagine this even in a worst-case scenario. We will do everything we can to support Ademi in this tough period for our player, who will need to stay as strong off the pitch as he's always been on it."
Dinamo coach Zoran Mamic added the club was shocked and intended to appeal the decision.
"To punish a player for four years after he and the experts proved that he took a contaminated supplement is senseless," he said. "Rules are rules and one must be careful what one consumes, but this is just too much because it's obvious he didn't know what he was doing.
"We will appeal to the Court of Sports Arbitration (CAS) or whoever is in charge but right now we are totally shocked."
Arsenal host Dinamo in the Champions League on Tuesday, with Arsene Wenger's side needing a win to keep alive any hopes of progressing to the last 16.
Dinamo's chief executive Zdravko Mamic was arrested on Wednesday for player transfer irregularities.
Both he and coach Zoran, his brother, were previously detained for questioning in July over embezzlement and tax evasion charges before being released.