What Are Heartworm Pills?

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  • Written By: Caitlynn Lowe
  • Edited By: Jessica Seminara
  • Last Modified Date: 26 October 2019
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Heartworm pills are one of the most common forms of preventative pet medication against the heartworm parasite. Dogs get heartworm disease more than other pets, but cats and other mammals are also at risk, especially if they spend time outdoors. Since heartworm disease is difficult to treat once it develops, veterinarians typically recommend taking measures to prevent it. Heartworm pills are both inexpensive and effective, making them a common preventative treatment, but other preventative treatments do exist.

Dirofilaria immitis, more commonly known as heartworms, transfer to household pets when an infected mosquito bites them. These parasites attack the heart and bloodstream of an infected animal. In 6 months or less, heartworms cause organ damage and failure, particularly in the lungs and heart. This damage occurs whether a pet is infected by several worms or several hundred.

Since heartworm disease is transmitted through mosquito bites, pets who spend time outdoors have the greatest risk of getting infested. Pets who live in warm, humid climates are especially prone to heartworm disease. Most heartworm infestations are found in dogs, but cats and ferrets are also susceptible. Those who own any of these pets should consult their veterinarians about a prescription for heartworm pills or another preventative treatment.


Obtaining heartworm pills requires a prescription from a veterinarian. The right dosage for a pet depends on the type of pet — dog, cat, or ferret — as well as the size or weight of the pet. A lighter weight dog must not take a prescription meant for a heavier dog, since this dosage will be too high and may cause health problems. Most brands are administered on a monthly basis. A veterinarian may suggest administering heartworm pills only during peak mosquito season, but many veterinarians will recommend administering the pill year-round to provide the ultimate level of protection.

Each brand uses a different drug to destroy heartworm larvae before it has a chance to mature and breed, but the most common drugs used in these medications are ivermectin and milbemycin oxime. Some dogs have a genetic sensitivity to ivermectin, however. A veterinarian may recommend testing a dog for this sensitivity if it has certain breeds in its bloodline, including but not limited to Collie, Shetland Sheepdog, and Australian Shepherd. If a dog tests positive, milbemycin oxime or another drug should be administered, instead.

While all heartworm pills prevent the heartworm parasite, some also prevent other parasites, such as hookworms, whipworms, and roundworms. Other factors that pet owners should consider include cost and reputation. All reputable brands should come with a manufacturer's or veterinarian's guarantee for safety and effectiveness.

Topical heartworm medication is a safe, effective alternative to heartworm pills. Most brands of topical heartworm medication use a drug called selamectin. In addition to medication, pet owners should consider taking measures to prevent or reduce the risk of mosquito bites. Individuals should keep their pets indoors throughout most of the late afternoon and evening, when mosquitoes are the most active. Removing sources of standing water and spraying mosquito repellent around the yard can also help reduce the mosquito population, reducing the overall threat to pets.


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