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

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!