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Mirtazapine is a drug that is used for appetite stimulation in cats. It has no effect on the GI tract as far as treating diarrhea and I am not sure why it was given to you for that reason, or why they would expect he would have diarrhea after a neuter. If he has diarrhea now, two weeks after the neuter, it is not due to the procedure and I would expect there is aome underlying GI disease. I would definitely follow up with your vet for further work up and treatment.
In the meantime however, I would fast Domino to give his GI system a rest. Since he is an adult cat, a 12-24 hr fast will not hurt him and will let his gut calm down. Continue to offer small amounts of water at a time. When you do begin feeding him again, you will want to offer frequent, small meals of something very digestible for him. These would include items like boiled hamburger or chicken. No sauces or gravies, but a little non greasy broth would be OK. If the stools improve, I would continue that for a few days and then slowly wean him back onto his normal food and feeding schedule.
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You are very welcome. Poor guy. I hope he is better by tomorrow.
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