Never use over the counter products on your pet. They are a lot cheaper i know, but they are weaker than your vet will supply and you do not know just what is in them. I buy Drontel from my vet if i need to worm. But recomend that you use the new flea treatment stronghold from the vet which includes ear mite control and tate wormer, all in one. you just put it on the back of their neck once a month. I now only worm them once a year after the summer, because fleas cause worms, so you break the cycle by killing any flea that comes in contact with your cat including eggs. These are what cause worms. If there are no fleas or eggs it stops them from getting worms. If any get through the treatment kills them in the stomach. Hope this is of some help.