Is frontline harmful to cats if they lick it?

It can be harmful. If you think your cat has ingested the product, call your Vet or the ASPCA Poison Control hotline. These types of products should be applied at the nape of the cat's neck where it is unable to lick it. If you have multiple cats that want to lick these products off each other, separate them for the day while the medication disperses through their fur and dries.