It doesn’t take long to notice something is very, very wrong… A few years ago my first pets (2 cats) had gotten fleas, and since they were my first animals I didn’t know what I was doing. I should have called my vet but I didn’t. I went to the grocery store and bought some Hartz flea & tick stuff you put on there backshoulders. Seemed easy enough. So I get home put in on them and they went NUTS! They started pawing at their heads, squinting their eyes, running as fast as they could through the house until throwing themselves onto the floor into a howling craze. I thought they were just being sissys about it, so I paid no real mind. Until 2 days later they wouldn’t snap out of it, not opening their eyes, not eating or anything. I took a wash cloth, washed it off and they snapped back. Coincidence? I don’t think so. I will never use their products again.