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Thank you for your question. What is wrong with Toby that you want to give Castoria?
I am a veterianrian and will do my best to help.
The active ingredient in Castoria is senna. It is not very effective for treating constipation in cats, if that is Toby's problem. It can cause nausea in some cats, but in low doses it should be safe.
So a taste of it should be safe, but not effective for treating constipation. If constipation is Toby's problem, I would try adding fiber to his diet (canned pumpkin, sweet potato), or Mira Lax, made for people but safe in cats at 1/4th teaspoon twice daily on the food.