Ivermectin (22,23-dihydroavermectin B1a + 22,23-dihydroavermectin B1b) is a broad-spectrum anti-parasite medication, traditionally used against worms (except tapeworms), but more recently found to be effective against most mites and some lice too. Ivermectin is used in horses for the treatment and control of large strongyles (adult) (Strongylus vulgaris, Strongylus edentatus, Triodontophorus species), small strongyles (adult and fourth stage larvae) (Cyathostomum species, Cylicocyclus species, Cylicostephanus species), pinworms (adult and fourth-stage larvae) (Oxyuris equi), large roundworms (adult) (Parascaris equorum), hairworms (adult) (Trichostrongylus axei), large mouth stomach worms (adult) (Habronema muscae), neck threadworms (microfilariae) (Onchocerca species), stomach bots (Gastrophilus species), and canine nasal mite infections.

In dogs and cats ivermectin is approved only as a preventative for heartworm. It has also been used as a microfilaricide, slow-kill adulticide, ectoparasiticide (to kill external parasites), and endoparasiticide (to kill internal parasites). In animals, ivermectin may cause local tissue swelling and central nervous system dysfunction. Death has been reported in some shelties and collies. Australian shepherds and Old English sheepdogs may also be sensitive to treatment with this drug. Treatment of dogs with heartworm often causes tremors, incoordination, seizures, gross dilation of the pupil, dehydration, and possibly death.