As long as you buy it from a vet or a reputable pet store/website. You need Revolution, Advantix or Frontline Plus. Never ever buy it OTC from the grocery stores or pet shops. Never ever use Hartz brands (they have killed animals). To go along with your worms question, tapeworms come from the dog swallowing a flea. If your dog has fleas, deworm him, if you see a tapeworm segment, treat for fleas too.