Heartgard Plus is one of the most recognized and widely prescribed heartworm prevention products for dogs. Manufactured by Boehringer Ingelheim, it combines ivermectin (a macrocyclic lactone that prevents heartworm disease) with pyrantel pamoate (which treats and controls roundworms and hookworms). Heartgard Plus is given as a flavored monthly chewable and requires a veterinary prescription. Generic alternatives such as Tri-Heart Plus are available, making it one of the more affordable heartworm prevention options on the market.
What Is Heartgard Plus Used For?
Heartgard Plus is prescribed to prevent heartworm disease caused by Dirofilaria immitis and to treat and control two common intestinal parasites:
- Heartworm prevention — eliminates the tissue stage of heartworm larvae (L3 and L4) transmitted by mosquitoes within the past 30 days
- Roundworms (Toxocara canis, Toxascaris leonina) — treats and controls active infections
- Hookworms (Ancylostoma caninum, Uncinaria stenocephala, Ancylostoma braziliense) — treats and controls active infections
Heartgard Plus does not protect against fleas, ticks, or tapeworms. Many pet owners pair it with a separate flea and tick preventative. Your veterinarian will recommend the best combination for your dog based on your geographic area and lifestyle.
How Does Heartgard Plus Work?
Heartgard Plus works through two complementary mechanisms:
- Ivermectin targets the nervous system of heartworm larvae by binding to glutamate-gated chloride channels, causing paralysis and death of immature heartworms before they can reach the heart and lungs. It works retroactively, killing larvae acquired during the previous 30 days.
- Pyrantel pamoate is a depolarizing neuromuscular blocking agent that paralyzes roundworms and hookworms in the intestinal tract, allowing them to be passed in the stool.
The ivermectin dose in Heartgard Plus (6 mcg/kg) is extremely low compared to doses used to treat conditions like mange. At this preventive dose, ivermectin is considered safe for all dog breeds, including those with the MDR1 (ABCB1) gene mutation commonly found in Collies, Australian Shepherds, and related herding breeds.
Dosage
Heartgard Plus is dosed by body weight. Your veterinarian will determine the correct product size for your dog. It is given once monthly, year-round in most regions.
| Dog’s Weight | Ivermectin per Chew | Pyrantel per Chew | Product Color |
|---|---|---|---|
| Up to 25 lbs (up to 11 kg) | 68 mcg | 57 mg | Blue |
| 26-50 lbs (12-22 kg) | 136 mcg | 114 mg | Green |
| 51-100 lbs (23-45 kg) | 272 mcg | 227 mg | Brown |
Dogs over 100 lbs may require a combination of chews as directed by your veterinarian. Give Heartgard Plus on the same date each month. If you miss a dose, give it as soon as you remember and contact your veterinarian, as a gap in coverage may allow heartworm infection to establish.
How to Give Heartgard Plus to Your Dog
- Heartgard Plus is a beef-flavored chewable designed to be given as a treat. Most dogs accept it willingly.
- Give once monthly, ideally on the same day each month.
- Can be given with or without food.
- Ensure your dog chews the tablet completely rather than swallowing it whole. If your dog spits out or vomits the chew within an hour, contact your veterinarian about re-dosing.
- A heartworm test is required before starting and typically annually thereafter, even if doses have not been missed.
- Do not split chews between dogs. Use the correct weight-based product for each individual dog.
Side Effects
Heartgard Plus is one of the best-tolerated heartworm preventives available. Side effects are uncommon at the labeled preventive dose.
| Common Side Effects | Serious Side Effects (Contact Your Vet) |
|---|---|
| Vomiting (rare) | Lethargy, incoordination, or tremors |
| Diarrhea or soft stool (rare) | Dilated pupils or blindness |
| Decreased appetite (rare) | Seizures |
| Drooling (rare) | Difficulty breathing |
| Collapse |
Serious neurological side effects are extremely rare at preventive doses but have been reported. Dogs with heavy heartworm burdens that receive any heartworm preventive may experience a serious reaction due to the rapid die-off of microfilariae. This is why a negative heartworm test is essential before starting prevention.
Drug Interactions
- Spinosad (Comfortis, Trifexis): Concurrent use has been associated with increased ivermectin levels and potential for neurological toxicity. Discuss timing with your veterinarian.
- P-glycoprotein inhibitors (ketoconazole, cyclosporine, others): May increase ivermectin blood levels. Use caution and inform your vet of all medications.
- Other macrocyclic lactones: Avoid doubling up on ivermectin, moxidectin, or similar compounds without veterinary guidance.
Contraindications
- Dogs that have not been tested for heartworm — administering preventives to heartworm-positive dogs can be dangerous
- Dogs with known hypersensitivity to ivermectin or pyrantel
- Puppies under 6 weeks of age
MDR1 (ABCB1) gene note: While dogs with MDR1 mutations (Collies, Australian Shepherds, Shelties, and related breeds) are at risk for ivermectin toxicity at high doses, the preventive dose in Heartgard Plus (6 mcg/kg) is well below the toxic threshold and is considered safe for these breeds. If you have concerns, genetic testing is available through the Washington State University Veterinary Clinical Pharmacology Laboratory.
Storage
- Store at controlled room temperature, 68-77degF (20-25degC).
- Protect from light and moisture.
- Keep in original packaging until use.
- Keep out of reach of children and pets (dogs may eat the entire box if accessible).
Cost & Availability
Heartgard Plus is a prescription medication available through veterinary clinics, licensed online pet pharmacies, and some retail pharmacies.
- Approximate cost: $45-$60 for a 6-month supply (medium-sized dog)
- Generic alternatives: Tri-Heart Plus and generic ivermectin/pyrantel chewables are available and typically cost 20-30% less
- Available sizes: Three weight ranges (up to 25 lbs, 26-50 lbs, 51-100 lbs)
- Many veterinary clinics offer rebates or multi-month discounts, especially when purchased for 12 months
Frequently Asked Questions
Can I give Heartgard Plus to my Collie or Australian Shepherd? Yes. The ivermectin dose in Heartgard Plus (6 mcg/kg) is far below the threshold that causes problems in MDR1-mutant dogs. The American Heartworm Society and veterinary pharmacologists confirm that heartworm prevention doses of ivermectin are safe for all breeds. Higher doses of ivermectin used for other conditions (such as mange treatment) are where MDR1-mutant breeds face serious risk.
What happens if I miss a dose? Give the missed dose as soon as you remember and resume your regular monthly schedule. If more than 45 days have passed since the last dose, contact your veterinarian. A heartworm test may be recommended 6-7 months later, as it takes that long for an infection to become detectable.
Does Heartgard Plus kill fleas and ticks? No. Heartgard Plus prevents heartworm and treats intestinal worms only. You will need a separate product for flea and tick protection. Many dog owners combine Heartgard Plus with an isoxazoline product like NexGard, Simparica, or Bravecto for comprehensive parasite coverage.
Is year-round prevention really necessary? The American Heartworm Society recommends year-round prevention regardless of climate. Mosquitoes can emerge during unseasonably warm periods, and gaps in coverage create risk. Many veterinarians also emphasize that the monthly intestinal deworming benefit provides additional protection throughout the year.
How does Heartgard Plus compare to all-in-one products like Simparica Trio? Heartgard Plus covers heartworm, roundworms, and hookworms. All-in-one products like Simparica Trio or NexGard Plus add flea and tick protection in one monthly chew. The tradeoff is cost and flexibility — Heartgard Plus is less expensive and allows you to choose your flea/tick product separately, while all-in-ones offer convenience. Your veterinarian can help you decide which approach is best.
Sources & References
- American Heartworm Society - Current Canine Guidelines
- FDA - Heartgard Plus Product Information
- Merck Veterinary Manual - Heartworm Disease
- Washington State University - MDR1 (ABCB1) Testing
- VCA Hospitals - Heartworm Prevention for Dogs
Verification Notes
Heartgard Plus pricing ($45-60 per 6-month supply for medium dogs) and generic availability (Tri-Heart Plus) are consistent with the research data. The MDR1 safety note aligns with the research corpus: “Ivermectin can be problematic in MDR1-mutant herding breeds; however, heartworm prevention doses are safe for all breeds.” The requirement for heartworm testing before starting prevention is supported by the fact-check report (Claim C25). The American Heartworm Society recommendation for year-round prevention is well-documented in veterinary guidelines.