I'd suggest going back to the vet and asking for a prescription for ondansetron, Becca, because it has a different mode of action in the body and might help when Cerenia doesn't. Ask them to give you guidance on what size of ondansetron dose you can give every 8-12 hours.
My civvie, Lúnasa, had two 30-day courses of co-amoxiclav antibiotic last year and she would never have got through them without support from ondansetron. Even with the anti-nausea med it was a struggle because the antibiotic gave her diarrhoea for weeks. (((The Noodle))) FYI, the dose that worked for her was 2mg q12h (10lb | 4.5kg cat). Sometimes a dose of 1mg q8h might work better.
If you do get an ondansetron Rx, then if at all possible give the ondansetron and wait about an hour or so to let the dose kick in before administering the antibiotic.
If at all possible, I'd recommend daily monitoring for ketones, especially if you're worried about whether it's safe to give insulin.
I hope Candi gets over the UTI really soon.
(((Candi)))
Mogs
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