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Heartworm Prevention for Dogs

By the Dog Health Handbook team · Updated · Fact-checked against current veterinary references (see sources)

Key takeaways

  • Give a prescription preventive year-round, on time, every time.
  • Choices: monthly chews, monthly topicals or a 6- or 12-month injection.
  • Start puppies by 8 weeks; test older dogs first.
  • Preventives are safe at labeled doses, including in MDR1 breeds.
  • No natural product is proven to prevent heartworm.

Table of Contents

Research Types Year-round Starting Safety Resistance Prescription Natural options FAQ Sources

Heartworm Quicklinks

Heartworm Symptoms How Dogs Get It Testing Treatment Prevention Other Worms

Heartworm prevention for dogs means giving a prescription preventive year-round, on time, every time. These medicines are in the macrocyclic lactone class and come as monthly chews or tablets, monthly topicals, or an injection that lasts 6 or 12 months.1,2 They work by killing heartworm larvae that infected your dog in the past month or so, before the larvae can grow into adults.1,3 The American Heartworm Society (AHS), the Companion Animal Parasite Council (CAPC) and the FDA all recommend year-round prevention for every dog.2,4,5 Puppies should start by 8 weeks of age.2 Dogs 7 months and older should be tested before starting, and every dog should be tested yearly.2,5 US heartworm preventives are prescription only, and none of the natural alternatives are proven to work.5,6 Prevention costs far less than treatment.7

Call your vet if:

  • You missed a dose or gave it late. Your dog may need testing now and in 6 months.3,8
  • Your dog vomited soon after an oral dose, or was bathed soon after a topical dose9
  • Your dog seems unwell after a dose. Your vet can report suspected side effects to the manufacturer.5,10

Never give livestock parasite products to dogs. Overdoses can cause serious poisoning.2

What the Research and Guidelines Say

  • Prevention works. In 83,478 tested dogs, 0.90% of those with no preventive purchases tested positive, compared with 0.32% of dogs with some monthly preventive purchases and 0.14% with some injectable prevention, even with gaps in coverage.11
  • Many dogs miss out. On average, owners bought only 7.3 months of monthly preventive doses per year.12
  • Resistance is spreading. A 2025 study of 310 samples from infected dogs found resistant or mixed resistant heartworms in many states, "as far north as Michigan."13
  • Cost. Prevention cost about $183 a year, compared with an average of $1,457 for treatment.7

Types of Heartworm Preventives

  • Monthly chewable tablets1
  • Monthly topical (spot-on) treatments1
  • Injections every 6 or 12 months, given by your vet1,2

Many products also control intestinal worms, fleas or ticks. Combination products are linked to more consistent use.12 All need a vet's prescription.1,5

Why Year-Round?

AHS recommends "year-round administration of preventive drugs," and the FDA agrees.2,8 Missing even one dose can allow infection.8 Mosquito seasons vary, and in warm areas transmission can happen all year.14 AVMA notes the risk "drops to almost zero when pets receive continuous, year-round heartworm protection."15 Indoor dogs need prevention too.16,15

Starting Prevention and Testing

  • Start puppies "as early as possible, no later than 8 weeks of age."2
  • Dogs 7 months and older need a heartworm test before starting or restarting prevention.2,5,15
  • Test every dog once a year, even when prevention is given correctly, because no product is 100% effective.2,17

Safety

AHS calls heartworm preventives "safe, relatively inexpensive and easy to give."1 Preventives are safe at labeled doses even in herding breeds with the MDR1 (ABCB1) gene mutation, such as Collies and Australian Shepherds.10,18,14 Problems in these dogs come from much higher doses, such as livestock products or some mange treatments.10,2 Ask your vet about possible interactions with other medicines.10

Resistance

AHS confirms that "ML-resistant heartworms have been documented." They were first found in the Mississippi Delta and are now found more widely.2,19,13 Even so, AHS says "owner compliance/adherence remains the biggest factor" when prevention fails.2 To strengthen protection, AHS recommends a multimodal approach:2,8

  • A heartworm preventive given on time, every time
  • A vet-recommended mosquito repellent and insecticide product for dogs
  • Reducing standing water and avoiding peak mosquito times
  • Yearly testing

Don't use insect repellents made for people on dogs unless your vet approves.2

Can I Buy Heartworm Prevention Without a Prescription?

No. The FDA says "heartworm prevention medications are prescription only" and warns against sellers that don't require a prescription.5 A vet's prescription ensures your dog is tested first and gets the right product and dose.5,15

Natural Alternatives

AHS reports that for nosodes and herbal preventives, "none of these products has been proven or FDA-approved to prevent heartworms in pets." Mosquito control alone "is not enough."6 Avoiding dusk and dawn walks and removing standing water can lower risk, but only alongside a preventive.6,8

Frequently Asked Questions

Does my dog really need heartworm prevention?

Yes. Heartworm is found in all 50 states and is far easier to prevent than to treat.

What happens if I miss a dose?

Call your vet, who will tell you when to give the next dose. Your dog may need testing now and in 6 months.

What is the safest heartworm preventive?

All FDA-approved preventives are safe at labeled doses. Your vet can help choose one.

Can I stop heartworm prevention in winter?

AHS recommends giving it year-round.

Are there natural ways to prevent heartworm?

No natural product is proven to prevent heartworm.

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Sources and further reading

This article was rewritten and fact-checked on October 24, 2026 against current veterinary references and specialist guidelines:

  1. American Heartworm Society. Heartworm Prevention for Dogs. American Heartworm Society.
  2. Nelson CT, McCall JW, Moorhead A, Starkey L, Ames M. Canine Guidelines for the Prevention, Diagnosis, and Management of Heartworm (Dirofilaria immitis) Infection in Dogs. American Heartworm Society, revised 2024.
  3. Moorhead A. Practical Advice About Heartworm Preventive Lapses. Today's Veterinary Practice, 2019.
  4. Companion Animal Parasite Council. CAPC Guidelines: Heartworm. CAPC, updated January 2026.
  5. US Food and Drug Administration. An Ounce of Prevention is Worth a Pound of Cure: Protect Your Pet from Heartworms Year-Round. FDA Consumer Update, 2023.
  6. American Heartworm Society. Are there natural ways to prevent heartworms? American Heartworm Society fact sheet.
  7. Mwacalimba K, Ferguson B, Brennan C, Poulsen Nautrup B, Little J, Stiefelmeyer K, Moldavchuk M, Mosko A, et al. Treatment of heartworm (Dirofilaria immitis) disease in dogs in the USA: general findings and cost of disease. Parasites & Vectors, 2026.
  8. American Heartworm Society. Canine Heartworm Guidelines: Highlights. American Heartworm Society, 2024.
  9. American Heartworm Society. Heartworm Preventive Resistance: Is it Possible? American Heartworm Society, 2013.
  10. Washington Animal Disease Diagnostic Laboratory. Are heartworm prevention products safe if my dog has the MDR1 mutation? Washington State University, 2021.
  11. Mwacalimba K, Morrison JA, Ly P, Spofford N, Yang M, Saito E, Sheehy J, Adolph C, Poulsen Nautrup B, Brennan C. Prevention and long-term outcomes of naturally occurring canine heartworm infection in primary care settings. Frontiers in Veterinary Science, 2024.
  12. Mwacalimba K, Sears D, Brennan C, Poulsen Nautrup B, Sheehy J, Smith K, et al. Retrospective analyses of heartworm (Dirofilaria immitis) disease and ectoparasite preventive medication compliance in veterinary practices in the USA. Parasites & Vectors, 2023.
  13. Curry E, Tack D, Rodriguez J, Brehm-Lowe D, Letherer J, Lineberry M, et al. Surveillance of single nucleotide polymorphisms correlated to macrocyclic lactone resistance in Dirofilaria immitis from client-owned dogs across the United States. International Journal for Parasitology: Drugs and Drug Resistance, 2025.
  14. Ames MK. Heartworm Disease in Dogs, Cats, and Ferrets. Merck Veterinary Manual, updated August 2026.
  15. American Veterinary Medical Association. Heartworm disease. AVMA.
  16. Atkins CE. Heartworm Disease in Dogs. Merck Veterinary Manual, Pet Owner Version, updated September 2024.
  17. American Heartworm Society. Heartworm Basics. American Heartworm Society.
  18. Mealey KL. Canine ABCB1 and macrocyclic lactones: heartworm prevention and pharmacogenetics. Veterinary Parasitology, 2008.
  19. Geary TG. Current issues in heartworm chemotherapy. Parasites & Vectors, 2026.

About this article

Last updated : written using the 2024 American Heartworm Society canine guidelines, 2026 Companion Animal Parasite Council recommendations, the FDA, the AVMA, the Merck Veterinary Manual and 2023 to 2026 research, plus International Veterinary Information Service (IVIS) proceedings and research published 2022 to 2026.

This page is for general information and is not a substitute for an exam by your veterinarian.