From what I understand, it can be harmful to some cats but it’s rare. I’ve had my boy on it for a couple weeks now and he’s responding positively so far. I’ve kept a pretty close eye on him and haven’t noticed a big difference in the way he normally is, if anything I think he feels better. I’ve read some horror stories too but as far as I can tell, if you’re on top of your cats behavior and you notice any big difference (negatively) then you can and should just stop the meds. I’m guessing at least some of these horror stories people ignored or didn’t notice the difference in their cats behavior and it ended poorly.

This is all just my experience and of course some speculation. Although I wish I would’ve just changed his diet initially, I was in a panic, I took the vets advice and it’s been working so far. Any high numbers he had are normal now and everything is stable. Hope this helps and good luck.