Quoting the Guardian, the former coach of Marin Chilic Bob Brett has confirmed that the world number 15 has indeed failed a drug test because of high gluclose which he said was cause by a product brought over the counter. The Guardian article quotes Brett as saying:-
"He said he had tested positive," said Brett. "The one thing I found out was that he tested positive for high glucose and Marin had co-operated with the organisation [the International Tennis Federation] about the product and the pharmacy. One of the people in his team bought it in a pharmacy – basically what you've been reading in the papers. Some people will say he made a mistake. But it was carelessness."
Source of information - http://www.theguardian.com/sport/2013/jul/30/marin-cilic-tennis-drug-test