Still have not located the drywall that supposedly came out of it. Not sure if he ate it (hoping not) or not.
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When she started walking, my youngest daughter uprooted and ate the roots from some of my pot plants (not THAT kind of pot plant) inside the house. The doctor called it Pica and said she must have some sort of vitamin or mineral deficiency, and to keep poison plants out of the house but otherwise sprout some wheat or food grains for her. At that time, 50 years ago, this old German doctor was against a lot of the other doctors' theories, but after letting her eat sprouted grains for a few months, she stopped ruining my favorite house plants.
Drywall is Lime, which my nurseryman husband used to put on the garden and all plants we have. I don't think it will hurt him.
Maybe your cat is the same, lacking some mineral in his diet?
You could buy him sprouted wheat in grocery stores, I get one for my cats every 10 days or so and they graze on it like goats!
Also, maybe a drop now and then of some kitty vitamins and minerals.
Should he continue this, besides checking with the vet, you could spray the area with Bitter Orange, the taste of which most cats hate.