I am going through the exact same process with one of my foster's right now. Bigodinho has a UTI and the vet put him on Clavamox drops (a type of penicillin). Sometimes it works to mix it in with the wet food, sometimes it doesn't so it is a very "hit and miss" proposition with this guy (my other cats are not like that ... they eat everything). I have even broken down and given him people tuna to see if that will help.

Good luck and you will notice a different within a couple of days, but make sure to use all the medication so the infection doesn't come back.

Cheryl