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Senior cat changed after methimazole. Meds, dementia, or anxiety?

Updated On September 23rd, 2025

Pet's info: Cat | American Shorthair | Male | neutered | 16 years and 4 months old | 9 lbs

Since getting my 16y.o. cat on the right dosage of methimazole he has COMPLETELY CHANGED, seems wiped out, doesn't rcome when I call, walks around like he's in a cloud, and stops mid-move like HE'S FORGOTTEN HIS INTENTION! Not sure if this is med-related, cognitive decline, or anxiety cuz 2 mos ago we moved into our own apt so he's alone daily & may be bored but won't play with toys anymore. He yowls randomly & sleeps in odd places. MD wants to start Prozac. Will this help him "come back?" HELP!

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2 Answers

Most Helpful Answer

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Answered By Lauren Jones VMD

Veterinarian

Published on August 18th, 2021

Richochet is one handsome little guy! I'm sorry to hear that he's had so many behavioral changes since his methimazole dose has been regulated. The most common side effects from methimzaole include stomach upset (vomiting, diarrhea, appetite changes), lethargy, facial itchiness, as well as some liver issues and changes in blood cell counts. Behavioral changes aren't documented as known side effects of this medication (I just double checked!), although excess vocalization has been recognized as a side effect. While we keep in mind that not all cats "read the textbooks," it would be unexpected for all of these behavioral changes to occur due to methimazole alone. Given your recent move and Richochet's age, it does stand to reason that anxiety from these changes or cognitive dysfunction (essentially kitty dementia) could be to blame here. I don't think it's necessarily wrong to give Prozac a try, but it's also important to note that it takes about 2 months for that medication to reach therapeutic levels. The other thing to consider would be discontinuing the methimazole (even temporarily) to see if his behaviors return to normal. Methimazole reaches a therapeutic level in about 2 weeks, so if you have to restart it, there's less of a time lag. Ultimately, I think there's going to be some trial and error involved here. Please remember that you should ONLY make any changes to Richochet's medications under the direct guidance of your vet. I hope that everything goes well!

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Other Answers

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    Answered By Dr. Strydom, DVM

    Veterinarian

    Published on

    Hello, thank you for using Petco Pet Education Center, formerly Petcoach. So sorry to hear that Ricochet is having this problem. Honestly, it is very difficult for us to say what will help or not in this particular situation without having all his medical records. I would say that typically we do not see these types of symptoms as a side effect of methimazole. I would also say that just based on the small amount of information I have, I don't think Prozac will help. If anything, I would expect it to make him more "down". If you are not aligning well with your regular veterinarian then I would recommend you take him to another vet for a second opinion. That would help you the most in my opinion.

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