Using flea control products wrongly can harm cats and dogs. If you use dog products on cats or apply them incorrectly, it can lead to issues. Cats can have bad reactions to some chemicals in these products, like pyrethrums and organophosphates. Signs of a problem include muscle shakes, drooling a lot, diarrhea, throwing up, trouble breathing, and feeling weak.If your pet seems sick from flea treatment, call your vet right away.