I just saw this thread and would like to chime in. One of my guys (Oliver) has been on Prozac for about 5 years now. We use the pill form and put it in the pill pockets. Like Cataholic I could see a difference within a couple of days. That being said, I could also see the difference when they changed brands (and it was not pretty ). So my advice to you is that it can make a HUGE difference and not all cats are on it as long as Oliver. But once the furry ones are stabilized on a dose and brand do not change them around (just like for humans).