Medications used to prevent heartworm infections are called preventives. Preventives kill the heartworm larvae before they have a chance to grow and mature into adult heartworms. A number of heartworm preventives are available to pet owners today. Some heartworm preventives contain additional ingredients that will control other parasites, such as roundworms or hookworms. A monthly preventive should be given to all pets. Remember that mosquitoes can get indoors, so even though your pet may not go outside, he is still susceptible.