Pica, an unnatural craving for abnormal things, is well documented in people and animals. And, it usually does tie in with a deficiency such as iron.
Blood work will usually reveal the more common causes for pica, and appropriate supplementation cures the problem. There are also psychological causes for abnormal ingestion of things such as cat litter. And, yes, eating cat litter can cause medical problems such as obstipation and heavy metal poisoning potentionally over time.
I would first change the litter type and make certain your kitty is on a balanced, nutrious diet, and additionally apply some environmental enrichment such as a window platform to address any boredom issues.
If pica continues, then a physical exam and directed diagnostics to clarify the problem.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124