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Interceptor vs. Heartgard: Which Is Right for Your Pet?
Published on August 15th, 2023
Updated on June 18th, 2026
Heartgard vs. Heartgard Plus vs. Interceptor: What's the Difference?
If you've searched "Heartgard vs. Heartgard Plus," you're not alone — and the distinction matters. Heartgard and Heartgard Plus are two separate products with different active ingredients and parasite coverage. Interceptor is a third option with its own formula. Here's a clear breakdown of all three so you can have an informed conversation with your veterinarian.
Heartgard vs. Heartgard Plus: The Key Difference
Heartgard (ivermectin only) prevents heartworm disease in dogs and cats. That's its primary job.
Heartgard Plus (ivermectin + pyrantel pamoate) does everything Heartgard does — and adds protection against intestinal parasites in dogs, specifically roundworms and hookworms. For many pet parents, Heartgard Plus is the more commonly prescribed version precisely because of that broader coverage.
Quick answer for featured snippet: Heartgard contains ivermectin alone and prevents heartworm. Heartgard Plus contains ivermectin and pyrantel pamoate, adding roundworm and hookworm protection for dogs. Both require a prescription and a negative heartworm test before starting.
Full Side-by-Side Comparison: Interceptor vs. Heartgard vs. Heartgard Plus
| Feature | Interceptor | Heartgard | Heartgard Plus |
|---|---|---|---|
| Species | Dogs & cats | Dogs & cats | Dogs only |
| Active ingredient(s) | Milbemycin oxime | Ivermectin | Ivermectin + pyrantel pamoate |
| Primary usage — dogs | Prevents heartworm (Dirofilaria immitis); protects against hookworm, roundworm and whipworm; controls adult hookworm; eliminates adult roundworm and whipworm infections | Prevents heartworm infection (Dirofilaria immitis) | Prevents heartworm infection (Dirofilaria immitis); treats and controls roundworms and hookworms |
| Primary usage — cats | Prevents heartworm (Dirofilaria immitis); removes adult hookworms and roundworms | Prevents heartworm infection; removes and controls hookworm infections (Ancylostoma tubaeforme and A. brasiliense) | Not approved for cats |
| Extra-label usage — dogs | Treats mite infestations | Treats demodectic mange, scabies, ear mites, hookworms, roundworms and capillaria | |
| Extra-label usage — cats | May treat ear mites and feline scabies | May treat ear mites and feline scabies | N/A |
| Available form | Tablet | Tablet | Chewable tablet |
| How it works | Derivative of soil microorganisms (Streptomyces genus); interferes with invertebrate neurotransmission | Derivative of soil microorganisms (Streptomyces genus); interferes with invertebrate neurotransmission | Same mechanism as Heartgard; pyrantel pamoate acts as a depolarizing neuromuscular blocking agent |
| Takes effect | 1–2 days for internal parasites | 1–2 hours | 1–2 hours |
| Approved for — dogs | 4 weeks and older; 2 lbs. or heavier | 6 weeks and older | 6 weeks and older |
| Approved for — cats | 6 weeks and older; 1.5 lbs. or heavier | 6 weeks and older | Not approved |
| Prescription required? | Yes | Yes | Yes |
| Heartworm test required first? | Yes | Yes | Yes |
| Safe for pregnant/nursing dogs? | Safety unknown; use with caution | Yes (per label) | |
| Breed considerations | Monitor Collies, Sheepdogs and Collie/Sheepdog crossbreeds for sensitivities | Monitor Collies, Sheepdogs and Collie/Sheepdog crossbreeds for sensitivities | Monitor Collies, Sheepdogs and Collie/Sheepdog crossbreeds for sensitivities |
| Potential side effects | Depression, lethargy, vomiting, ataxia, anorexia, diarrhea, convulsions, weakness, hypersalivation | Stomach upset, vomiting, diarrhea, dilated pupils, unsteadiness, dazed demeanor; neurological effects and shock possible at high doses; adverse reactions at lower doses in Collie and Sheep breeds | Similar to Heartgard; contact your veterinarian if any side effects appear |
| Adverse drug interactions | Cyclosporine, amiodarone, diltiazem, azole antifungals, erythromycin | Ketoconazole, itraconazole, cyclosporine, erythromycin, amlodipine besylate, nifedipine |
Monthly Dosage Guidelines
Dosing is weight-based for all three products. Your veterinarian will confirm the right dose for your dog or cat at each visit.
Interceptor — Monthly Dosage
Dogs:
- 2–10 lbs. → 1 tablet (2.3 mg)
- 11–25 lbs. → 1 tablet (5.75 mg)
- 26–50 lbs. → 1 tablet (11.5 mg)
- 51–100 lbs. → 1 tablet (23.0 mg)
Cats:
- 1.5–6 lbs. → 1 tablet (5.75 mg)
- 6.1–12 lbs. → 1 tablet (11.5 mg)
- 12.1–25 lbs. → 1 tablet (23.0 mg)
Heartgard — Monthly Dosage
Dogs:
- Up to 25 lbs. → 1 tablet (68 mcg ivermectin)
- 26–50 lbs. → 1 tablet (136 mcg ivermectin)
- 51–100 lbs. → 1 tablet (272 mcg ivermectin)
Cats:
- Up to 5 lbs. → 1 tablet (55 mcg ivermectin)
- 6–15 lbs. → 1 tablet (165 mcg ivermectin)
Heartgard Plus — Monthly Dosage
Heartgard Plus is dosed by ivermectin + pyrantel pamoate content.
Directions for Use
- Interceptor — dogs: Give orally once a month at the recommended minimum dosage rate of 0.23 mg per pound of body weight, with or without food.
- Interceptor — cats: Give orally once a month at the recommended minimum dosage rate of 0.9 mg per pound of body weight, with or without food.
- Heartgard and Heartgard Plus — dogs and cats: Give orally once a month, with or without food.
Which One Is Right for Your Pet?
The right choice depends on your dog's or cat's individual health history, lifestyle, geographic location and your veterinarian's recommendation. A few practical points to keep in mind:
- Both Heartgard and Heartgard Plus require a negative heartworm test before starting — never begin a preventive without that test, as treating an active heartworm infection requires a completely different protocol.
- Heartgard Plus is often preferred for dogs because the added pyrantel pamoate addresses roundworms and hookworms in a single monthly dose — parasites that are common and can be transmitted to people.
- Interceptor covers whipworm in addition to heartworm, hookworm and roundworm — a distinction worth discussing with your veterinarian if whipworm is a concern in your area.
- Breed sensitivity is real. Collies, Sheepdogs and their crossbreeds can carry a genetic mutation (MDR1/ABCB1) that affects how they process certain antiparasitic drugs. Always screen and monitor these breeds before starting any of these medications.
- Drug interactions matter. Share a complete list of your pet's current medications and supplements with your veterinarian before starting any new preventive.
If you ever notice side effects after giving any of these medications — vomiting, lethargy, unsteadiness or neurological changes — contact your veterinarian right away. For suspected poisoning or overdose, reach out to the ASPCA Animal Poison Control Center https://www.aspca.org/pet-care/animal-poison-control.
Always consult your veterinarian before starting, switching or stopping any parasite prevention medication. These are prescription products for good reason — the right choice is the one your veterinarian recommends for your pet's specific needs.
Frequently Asked Questions
What is the difference between Heartgard and Heartgard Plus? Heartgard (ivermectin only) prevents heartworm disease in dogs and cats. Heartgard Plus adds pyrantel pamoate to the ivermectin base, broadening coverage to also treat and control roundworms and hookworms in dogs — making it a combination product rather than a single-parasite preventive. If your dog needs intestinal parasite coverage alongside heartworm prevention, your veterinarian may recommend Heartgard Plus over standard Heartgard.
Does Heartgard or Heartgard Plus require a prescription? Yes — both formulations are prescription-only medications. Your dog or cat must be examined by a veterinarian and tested negative for an existing heartworm infection before either product can be dispensed. Starting a preventive on a heartworm-positive pet can cause serious adverse reactions.
How does Heartgard compare to Interceptor for intestinal parasite coverage? Interceptor (milbemycin oxime) covers heartworm, hookworm, roundworm and whipworm in dogs, and heartworm and hookworm in cats. Standard Heartgard (ivermectin) covers heartworm in both species, plus hookworm in cats. Heartgard Plus adds roundworm and hookworm coverage for dogs but still does not address whipworm — a gap that Interceptor fills. Discuss your pet's specific parasite risk with your veterinarian to choose the right fit.
Are Collie-breed dogs at higher risk with ivermectin-based products like Heartgard? Yes. Collies, Shetland Sheepdogs, Australian Shepherds and related crossbreeds can carry an MDR1 (ABCB1) gene mutation that impairs their ability to clear certain drugs, including ivermectin, from the brain — potentially causing neurological side effects even at doses considered safe for other breeds. Your veterinarian can screen for this mutation before prescribing any ivermectin-containing product.
Can Heartgard or Interceptor be given to pregnant or nursing dogs? Heartgard (ivermectin) is labeled as acceptable for use in pregnant and nursing dogs. Interceptor (milbemycin oxime) carries a caution — its safety in pregnant or nursing dogs has not been fully established, so use should be guided by your veterinarian. Neither product should be started or continued without veterinary oversight during pregnancy or lactation.
What should I do if my pet shows side effects after taking a heartworm preventive? Contact your veterinarian immediately if you notice vomiting, diarrhea, lethargy, unsteadiness, dilated pupils or a dazed demeanor after dosing. For suspected poisoning or toxic reactions, you can also reach the ASPCA Animal Poison Control Center for guidance. https://www.aspca.org/pet-care/animal-poison-control Do not give another dose until your veterinarian has evaluated your pet.
This article was created with assistance from AI tools and reviewed for accuracy by Petco's content team.
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FAQs About Interceptor vs. Heartgard
No, Heartgard does not prevent whipworms. If you are looking for whipworm medicine for dogs, ask your vet about Interceptor Plus for dogs or another recommendation.
The chart above lists many similarities and differences between Interceptor vs. Heartgard. The primary difference between the two medicines is that Interceptor helps prevent or treat several conditions, including heartworms. Heartgard is primarily used only to prevent heartworms, though it can be used off-label to treat other conditions.
Many heartworm medicines for dogs require a prescription, so your first stop will be your vet’s office. After receiving authorization, you can have it filled at your vet’s office or most pet pharmacies. At Petco, our pharmacy fulfilled by Vetsource can fill many prescriptions, including Heartgard Plus, other heartworm medicines and dewormer treatments.
The American Heartworm Society recommends that all dogs receive year-round heartworm prevention treatment. Previously, heartworms were rarely seen in certain parts of the country, but cases have now been reported in all 50 states.
Dogs contract heartworms from infected mosquitos. The parasite Dirofilaria immitis will live and breed inside an infected dog’s lungs and heart and can grow up to 12 inches long. If left untreated, heartworms can cause lung, organ and heart damage by blocking blood vessels. Eventually, heartworm infection can be fatal.
Ultimately, it is up to you to determine if you feel your pet should receive heartworm medicine for dogs. Keep in mind that it’s much easier to prevent heartworms than to cure an active infection. Consult your veterinarian for more information on heartworm medicine for dogs.