Heartgard Plus Review (2025): Heartworm Prevention for Dogs

Affiliate disclosure: ReviewPooch earns commission on purchases made through links in this article, at no cost to you.

Heartgard Plus (ivermectin/pyrantel) has been the most widely used heartworm preventative in the US for decades. It's a monthly beef-flavored chewable that prevents heartworm disease and treats roundworms and hookworms. It doesn't cover fleas, ticks, or whipworms — so whether it's the right choice depends on what other protection your dog already has.

What Is Heartgard Plus?

Heartgard Plus is a prescription monthly chewable manufactured by Boehringer Ingelheim. Two active ingredients: ivermectin at 6 mcg/kg (kills heartworm larvae) and pyrantel pamoate at 5 mg/kg (treats roundworms and hookworms).

Approved by the FDA. Available in three weight-based sizes: up to 25 lbs (blue), 26–50 lbs (green), 51–100 lbs (brown). The chewable formulation is beef-flavored — most dogs take it readily as a treat.

Requires a prescription in the US. Vets typically ask for a heartworm test before prescribing because starting ivermectin-based prevention in a dog that's already infected with adult heartworms can cause a severe reaction.

What It Covers — and What It Doesn't

If your dog has whipworm exposure risk, you'll want to look at Interceptor Plus (milbemycin oxime) instead, which covers whipworms. If you want everything in one chew, Simparica Trio or NexGard Plus adds flea and tick coverage on top of heartworm prevention.

How It Works

Ivermectin works by eliminating the tissue-stage larvae (L3 and L4) of Dirofilaria immitis that a dog acquires from mosquito bites during the previous 30 days. It does not kill adult heartworms already established in the heart and pulmonary arteries — that's a separate, much more intensive treatment. Heartgard Plus prevents new infections; it doesn't clear existing ones.

This is why the 30-day dosing interval matters. Ivermectin provides roughly 30 days of larval kill activity. If a dose is missed and a dog is in a heartworm-endemic area, larvae from the gap period can mature past the susceptible stage. A dog that misses doses for several months may need to be re-tested before restarting prevention.

Pyrantel pamoate paralyzes roundworms and hookworms by blocking neuromuscular transmission, allowing the dog's digestive system to expel them. It works against adult parasites, not larvae.

Safety and Side Effects

Heartgard Plus has an extensive clinical safety record. In controlled field trials, adverse effects were observed in approximately 1.1% of administered doses — primarily vomiting or diarrhea within 24 hours of dosing.

Serious adverse events are rare. Reported post-market effects include itching, digestive symptoms, and in uncommon cases, neurological signs (ataxia, lethargy, dilated pupils). These typically resolve once the medication is cleared.

The FDA prescribing information notes that at the recommended dose (6 mcg/kg ivermectin), no adverse reactions occurred in sensitive Collie dogs or in standard breeds. The margin between the therapeutic dose and the dose that causes toxicity in dogs is large — the product was tested at 10 times the recommended dose in Collies without toxicity.

Collies and MDR1-Sensitive Breeds

The concern with ivermectin and herding breeds stems from the ABCB1-1Δ gene mutation (formerly called MDR1), which affects the blood-brain barrier's ability to pump certain drugs out of the central nervous system. Affected dogs accumulate higher concentrations of ivermectin in the brain at doses that would be harmless in normal dogs.

Breeds at elevated risk: Rough Collie, Smooth Collie, Border Collie, Australian Shepherd, Shetland Sheepdog, Old English Sheepdog, and some mixed breeds.

At the dose used in Heartgard Plus (6 mcg/kg), Boehringer Ingelheim's own toxicity studies showed no adverse effects in sensitive Collies — this is significantly lower than the dose at which MDR1-sensitive dogs show toxicity (around 100 mcg/kg and above). Current veterinary consensus is that Heartgard Plus is safe for MDR1-positive dogs at the labeled dose.

If you have an affected breed and want additional certainty, a genetic test from Washington State University's Veterinary Clinical Pharmacology Lab can confirm your dog's MDR1 status.

Dosing and Administration

Give once monthly, on or about the same date each month. Can be given year-round or seasonally depending on your region — vets in year-round mosquito climates (most of the southern US, Hawaii) typically recommend year-round dosing.

Minimum age: 6 weeks. Minimum weight: 1.5 lbs (the smallest approved dose). No upper age limit; dogs on ongoing heartworm prevention can continue Heartgard Plus throughout their lives.

If your dog vomits within an hour of taking the chew, redose with another tablet. If vomiting is consistent, giving the chew with food typically resolves it.

Dogs that test positive for adult heartworms should not be started on ivermectin-based prevention until after completing melarsomine (Immiticide) treatment to kill the adult worms. The immune reaction triggered by killing microfilariae (larval heartworms in the blood) in a heavily infected dog can be serious.

Heartgard Plus vs Interceptor Plus

If your dog has had whipworm infections or lives in an area with high whipworm burden, Interceptor Plus is the more complete option. For most dogs without that specific risk, Heartgard Plus covers the base heartworm + GI parasite needs at a slightly lower cost.

Heartgard Plus vs Simparica Trio

Simparica Trio adds sarolaner (an isoxazoline) to moxidectin and pyrantel — covering fleas, five tick species, and heartworm in one chew. The coverage gap vs Heartgard Plus is substantial: Simparica Trio includes flea and tick prevention that Heartgard Plus completely lacks.

The cost reflects it. Simparica Trio runs $36–42/month depending on dog size, versus $12–14 for Heartgard Plus. If you're already buying separate flea/tick prevention, the total cost of Heartgard Plus + a flea/tick product can approach or exceed Simparica Trio pricing — in which case consolidating makes sense. If tick exposure is low and you're using an OTC flea topical, Heartgard Plus is the more cost-efficient route.

See our Simparica Trio review for a full breakdown of the combination product.

Cost and Where to Buy

US vet clinic pricing: $70–90 for a 6-month supply depending on dog size. Independent pet pharmacies like 1-800-PetMeds and Chewy Pharmacy run slightly lower.

Canada Pet Care typically prices Heartgard Plus 25–35% below US vet clinic rates. A 6-month supply for a medium dog (26–50 lbs) that costs $85 at a US vet often runs $55–65 through Canada Pet Care on the same Boehringer Ingelheim manufacturer stock. You need a valid prescription, which your vet should provide on request — vets are legally required to release prescriptions to patients.

Verdict

Heartgard Plus does what it's designed to do with a strong safety record and minimal side effects. Ivermectin has been used in dogs for heartworm prevention for over 40 years, and the clinical evidence base is extensive. For a dog that just needs heartworm prevention plus basic roundworm/hookworm coverage, it's a reliable, affordable option.

The trade-off is narrow coverage. No fleas, no ticks, no whipworms. Dogs in tick-heavy regions will need a separate product. If you're already managing that cost, look at whether Simparica Trio or NexGard Plus consolidates things at a competitive total price.

Score: 8.5/10 — excellent efficacy and safety profile, but the narrower parasite coverage keeps it from being the top pick for dogs with multi-parasite exposure.

Frequently Asked Questions

Does Heartgard Plus kill adult heartworms?

No. Heartgard Plus kills heartworm larvae — L3 and L4 stage — acquired in the previous 30 days. It does not kill adult worms already established in the heart and pulmonary arteries. Dogs with existing adult heartworm infections require a separate adulticidal treatment protocol before starting prevention.

Can I give Heartgard Plus to a heartworm-positive dog?

Not before testing and treating. Starting ivermectin in a heartworm-positive dog can trigger a severe reaction. Vets require a negative heartworm antigen test before prescribing. If your dog tests positive, adulticidal treatment comes first.

How does Heartgard Plus compare to Interceptor Plus?

Both prevent heartworm and treat roundworms and hookworms. Interceptor Plus adds whipworm and tapeworm coverage and uses milbemycin oxime, which is generally considered safer for MDR1-sensitive breeds (collies, shelties, Australian shepherds). If your dog carries the MDR1 mutation, discuss Interceptor Plus with your vet.

Is Heartgard Plus safe for collies?

At the labeled dose (6 mcg/kg ivermectin), Heartgard Plus is generally considered safe even for MDR1-mutant dogs — the dose is below the toxicity threshold for sensitive breeds. Many vets still recommend milbemycin-based products like Interceptor Plus for confirmed MDR1-positive dogs as an extra precaution.

Where can I get Heartgard Plus without paying vet clinic prices?

Canada Pet Care stocks Heartgard Plus at 25–35% below US vet clinic pricing on the same Boehringer Ingelheim stock. You need a valid prescription, which your vet is legally required to release on request. See our Canada Pet Care review for details.

Ad — we may earn a commission at no extra cost to you.

Pros & Cons

Related Reviews