“I will be appealing that decision to the Court of Arbitration for Sport very shortly,” Cilic said in statement through his country’s tennis association.
“I wish to emphasise that I have never knowingly or deliberately taken any banned substance in my life and that I am opposed to any use of performance-enhancing substances in sport.”
The suspension has been backdated to May 1 to allow Cilic to return to the ATP circuit on Jan. 31 next year.
The ITF said Cilic had tested positive at the BMW Open in Munich in April this year.
“Unbeknownst to me, the glucose tablets contained a substance that is banned in competition although it is allowed out of competition,” Cilic added.
Cilic has not played since withdrawing from his second-round match at Wimbledon in June but retains a ranking of 24th in the world.
The tall righthander reached a career-high ranking of ninth in the world in February 2010, spending 11 successive weeks in the top 10.