The 24-year-old Cilic, who reached a career-best ranking of No. 9 in 2010, has not played since pulling out from Wimbledon in June before a second-round match, citing a left knee injury. He has since missed the U.S. Open and will also be ineligible for the Australian Open in January.
The ITF said Cilic will lose all his results, ranking points and prize money since the Munich tournament.
His best result since Munich was a runner-up finish at the Queen's Club grass-court tournament, where he lost to Andy Murray.
Cilic said in a statement he will appeal to the Court of Arbitration for Sport in Lausanne, Switzerland.
He said the banned substance came from a glucose tablet purchased at a pharmacy in France.
"Unbeknownst to me, the glucose tablets contained a substance that is banned in-competition (although it is allowed out-of-competition),'' he said.
"I wish to emphasize that I have never knowingly or deliberately taken any banned substances in my life and that I am opposed to any use of performance-enhancing substances in sport,'' Cilic added.
Cilic said he isn't able to comment further until the legal process is completed.
- AP