I worried for a long time (postponed the inevitable) because I thought it would make Jeddie a "zombie cat" with no personality except spaced out. He is on it for peeing all over except int he box and I wish I had tried it when it was first suggested to me here. I tried other things that didn't work (like Feliway and products like it).
Not only did the prozac work starting the next day, but if anything it has made Jeddie more of himself, if that makes any sense. It took the edge off of his anxiety that I didn't even know was there (well, I knew something was bothering him). He is sweeter, interacts more with the other cats, is more at ease, etc.
He was rehomed from his home of ten years where he was bottle fed as a tiny kitten, so he was angry, hurt and depressed. Can you blame him? As much as I made a good home for him, he still had issues that made him pee on the door, or the carpet, or in the bathtub (not a biggie, but not "right"), in the tray under the litter box...
FYI, when I asked my vet about it she didn't know it could be used for cats with inappropriate peeing issues. But she trusted that I had done my homework and let me try it (I had already tried it because someone here sent me 6 pills). I absolutely don't like giving unnecessary meds, but this really was a "magic" pill for him.