Vet called me back today she was very nice asked what potting soil I used, looked it up for me said it's supposed to be non-toxic and like someone said here they may be lacking something in their diet not likely though given I only use band name cat food even told thier vet maybe once a month they get 1/4 of spoonful of yougurt (they love it) she said not a problem, did they ack when eating grass (indoor guys) she suggested using more catnip and there a product out you put in the siol that will deter them from eating it, costs $11

right now I can't afford it but even in NY State I thnk safely put that one plant outside on my balconey only other plants I have are catci (sp)
Instead of buying something new for your soil, you can use aluminum foil to cover the soil, or stick pinecones or even plastic forks so the tines are facing up.
I am glad you talked to the vet.