Stronghold (selamectin) is used to treat parasitic infections in dogs. It is a spot-on solution applied to the skin, and it works by killing or paralyzing parasites such as fleas, ticks, mites, biting lice, ear mites, heartworms, roundworms and hookworms. Active ingredient, Selamectin, is an antiparasitic medicine that affects the nervous system of parasites, leading to paralysis or death.