Heartgard Plus for Dogs: Complete Dosage, Safety & Cost Guide
Complete guide to Heartgard Plus for dogs: dosage by weight, side effects, drug interactions, cost, and alternatives. Vet-reviewed safety information.
What Is Heartgard Plus and What Is It Used For in Dogs?
Heartgard Plus is a monthly oral chewable medication manufactured by Boehringer Ingelheim (formerly Merial) that prevents heartworm disease and treats two common intestinal parasites in dogs. It combines two active ingredients — ivermectin and pyrantel pamoate — into a single beef-flavored chewable tablet that most dogs readily accept as a treat.
The FDA-approved veterinary uses for Heartgard Plus include:
- Heartworm prevention: Eliminates the tissue larval stages of Dirofilaria immitis (heartworm) acquired in the preceding 30 days, preventing adult worms from establishing in the heart and pulmonary arteries
- Roundworm treatment and control: Effective against Toxocara canis and Toxascaris leonina
- Hookworm treatment and control: Effective against Ancylostoma caninum
Heartgard Plus is not approved for treating whipworms (Trichuris vulpis), tapeworms, fleas, ticks, or mites. If your dog needs broader parasite coverage, your veterinarian may recommend a different product or a combination protocol.
How Does Heartgard Plus Work? Mechanism of Action
Ivermectin Component (6 mcg/kg per dose)
Ivermectin belongs to the macrocyclic lactone drug class. It works by binding to glutamate-gated chloride ion channels found in invertebrate nerve and muscle cells. This binding causes increased permeability to chloride ions, leading to hyperpolarization of the cell membrane, paralysis, and death of the parasite. In dogs, ivermectin at the low preventive dose (6 mcg/kg) selectively targets the L3 and L4 larval stages of Dirofilaria immitis that were transmitted by mosquito bite within the preceding month. It does not kill adult heartworms — this is critical, as attempting to kill adult worms with preventive-dose ivermectin is ineffective and can create a false sense of security.
Pyrantel Pamoate Component (5 mg/kg per dose)
Pyrantel pamoate is a tetrahydropyrimidine anthelmintic that acts as a depolarizing neuromuscular blocking agent in susceptible parasites. It causes sudden contraction of the worms' muscles followed by permanent spastic paralysis. The paralyzed worms lose their grip on the intestinal wall and are expelled through normal peristalsis in the feces. Pyrantel is effective against luminal (adult) stages of roundworms and hookworms but does not affect migrating larval stages or parasites outside the gastrointestinal tract.
Because these two drugs target different parasite classes through entirely different mechanisms, they work synergistically without interfering with each other — which is why the combination is so widely prescribed.
Heartgard Plus Dosage by Dog Weight
Heartgard Plus comes in three weight-based formulations. The dosage is fixed within each weight bracket — do not split or combine tablets to create custom doses. Your veterinarian will prescribe the correct formulation based on your dog's current weight.
| Product | Dog Weight Range | Ivermectin per Chewable | Pyrantel Pamoate per Chewable | Ivermectin Dose (mcg/kg) |
|---|---|---|---|---|
| Heartgard Plus Green | Up to 25 lb (0–11.3 kg) | 68 mcg | 57 mg | ~6 mcg/kg |
| Heartgard Plus Brown | 26–50 lb (11.4–22.7 kg) | 136 mcg | 114 mg | ~6 mcg/kg |
| Heartgard Plus Gray | 51–100 lb (22.8–45.4 kg) | 272 mcg | 227 mg | ~6 mcg/kg |
For dogs over 100 lb (45.4 kg): Your veterinarian will prescribe the appropriate combination of tablets. For example, a 130 lb dog may receive one Gray (51–100 lb) tablet plus one Brown (26–50 lb) tablet. Never guess — have your vet calculate the combination.
Administration Instructions
- Frequency: Once every 30 days, year-round (the American Heartworm Society recommends 12-month prevention regardless of climate)
- With or without food: Can be given with or without food. Most dogs accept the beef-flavored chewable readily. If your dog refuses it, try hiding it in a small amount of peanut butter (xylitol-free), cheese, or a pill pocket
- Chewable format: The tablet is designed to be chewed, not swallowed whole. For dogs that gulp treats, break it into pieces and ensure chewing occurs
- After vomiting: If your dog vomits within 1 hour of administration, contact your veterinarian. A repeat dose may be recommended
- Testing requirement: Dogs 7 months or older must have a negative heartworm antigen test before starting Heartgard Plus. Annual testing is required to maintain the manufacturer's guarantee
Side Effects of Heartgard Plus: What to Watch For
Heartgard Plus has a strong safety profile when administered at the labeled preventive dose. The ivermectin dose (6 mcg/kg) is approximately 1/40th of the dose that causes toxicity in most dogs. However, side effects can occur, and certain breeds carry a genetic sensitivity that increases risk.
| Side Effect | Estimated Frequency | Severity | Action Required |
|---|---|---|---|
| Vomiting | ~1–2% of doses | Mild | Monitor; call vet if repeated |
| Diarrhea | ~1–2% of doses | Mild | Monitor; usually self-resolving within 24 hours |
| Lethargy / decreased activity | <1% of doses | Mild to moderate | Monitor; contact vet if lasting >24 hours |
| Loss of appetite | <1% of doses | Mild | Monitor next meal; call vet if persists |
| Staggering / ataxia | Rare | Serious | Contact vet immediately |
| Tremors / seizures | Very rare | Serious | Emergency vet visit |
| Dilated pupils / blindness | Very rare | Serious | Emergency vet visit |
| Hypersalivation (excessive drooling) | <1% of doses | Mild | Usually self-resolving; monitor |
The MDR1 Gene Mutation: A Critical Breed Consideration
The MDR1 (ABCB1) gene mutation affects the blood-brain barrier's ability to pump ivermectin out of the central nervous system. Dogs homozygous for this mutation (two copies) are significantly more sensitive to ivermectin toxicity. Importantly, at the standard preventive dose of 6 mcg/kg, Heartgard Plus has been shown to be safe even in MDR1-mutant dogs — the toxic threshold for sensitive dogs is typically above 100 mcg/kg. However, this safety margin narrows considerably if dosing errors occur or if the dog ingests additional ivermectin from other sources.
Breeds with known MDR1 mutation prevalence include:
- Collies (~70% carry at least one copy)
- Australian Shepherds (~50%)
- Miniature American Shepherds (~50%)
- Shetland Sheepdogs (~15%)
- English Shepherds (~15%)
- German Shepherd Dogs (~5–10%)
- Mixed-breed dogs (variable, estimated ~2–5%)
The Washington State University Veterinary Clinical Pharmacology Laboratory offers MDR1 genetic testing. If your dog belongs to an at-risk breed, discuss testing with your veterinarian before starting any ivermectin-containing product.
Contraindications: Which Dogs Should NOT Receive Heartgard Plus
While Heartgard Plus is safe for the vast majority of dogs, certain conditions and situations require alternative preventive strategies:
- Heartworm-positive dogs: Heartgard Plus is a preventive, not a treatment for adult heartworm infection. Administering it to a dog with adult heartworms can cause a reaction as microfilariae (baby worms circulating in the blood) die rapidly. All dogs must test negative for heartworm antigen before starting prevention. If your dog tests positive, your veterinarian will follow the American Heartworm Society adulticide treatment protocol instead
- Puppies under 6 weeks of age: Heartgard Plus is not approved for puppies younger than 6 weeks. For very young puppies, your vet will advise when to begin prevention
- Known hypersensitivity: Dogs with a documented allergic reaction to ivermectin or pyrantel pamoate should not receive this product
- Severely debilitated or compromised dogs: Your veterinarian may delay preventive administration in dogs with significant systemic illness until the dog is stable
- Dogs receiving high-dose ivermectin from another source: Concurrent use of other ivermectin products (e.g., for mange treatment) creates additive dosing and increases toxicity risk
Drug Interactions with Heartgard Plus
While ivermectin at preventive doses has relatively few clinically significant drug interactions, several medications can increase the risk of ivermectin crossing the blood-brain barrier and causing neurotoxicity. Always inform your veterinarian of every medication, supplement, and over-the-counter product your dog receives.
| Interacting Drug | Drug Class | Interaction Mechanism | Risk Level |
|---|---|---|---|
| Spinosad (Comfortis, Trifexis) | Insecticide | May increase ivermectin levels; FDA noted elevated risk when combined at high ivermectin doses | Moderate — safe at preventive dose but monitor |
| Amlodipine | Calcium channel blocker | May inhibit P-glycoprotein, increasing CNS ivermectin exposure | Low to moderate |
| Ketoconazole / Itraconazole | Azole antifungals | P-glycoprotein inhibitors; may increase ivermectin CNS penetration | Moderate |
| Cyclosporine | Immunosuppressant | P-glycoprotein inhibition; increased ivermectin brain levels | Moderate to high |
| Erythromycin | Macrolide antibiotic | Competes for P-glycoprotein transport | Low to moderate |
| Other macrocyclic lactones | Antiparasitics (e.g., moxidectin, selamectin) | Additive CNS depression and toxicity risk | High — do not combine |
Critical note: Never administer two different heartworm preventives simultaneously. If you're switching from one product to another, follow your veterinarian's transition protocol to avoid overlapping doses.
How Long Does My Dog Need to Stay on Heartgard Plus?
The standard recommendation from the American Heartworm Society is year-round, lifelong prevention for all dogs in all 50 U.S. states. Here's why continuous prevention matters:
- Mosquito unpredictability: Heartworm is transmitted exclusively by mosquitoes. Even in cold climates, unseasonably warm winter days can produce mosquito activity. Heartworm larvae can develop to the infective L3 stage inside mosquitoes at temperatures as low as 57°F (14°C)
- Retroactive protection gap: Heartgard Plus kills larvae acquired in the preceding 30 days. If you stop in October and restart in April, larvae acquired in November could mature to adults before your April dose has any effect
- Intestinal parasite continuity: The pyrantel component provides monthly deworming for roundworms and hookworms — parasites your dog can acquire year-round from contaminated soil, other dogs' feces, or prey animals
- Manufacturer guarantee: Boehringer Ingelheim offers a heartworm guarantee that covers treatment costs if a dog tests positive while on Heartgard Plus — but only if the product was administered every 30 days without interruption and annual testing was documented
Missed dose protocol: If you miss a monthly dose, give Heartgard Plus as soon as you remember and resume the 30-day schedule from that date. If the gap between doses exceeds 45 days, your veterinarian may recommend a heartworm antigen test before resuming, particularly if more than 2 months have elapsed or if you live in a high-prevalence area.
Overdose response: If your dog accidentally ingests multiple Heartgard Plus chewables (for example, gets into the box), contact your veterinarian or the ASPCA Animal Poison Control at (888) 426-4435 immediately. While the preventive dose has a wide safety margin, ingestion of multiple tablets can push ivermectin levels into the toxic range, especially in small dogs or MDR1-sensitive breeds. Symptoms of overdose include dilated pupils, disorientation, tremors, drooling, vomiting, and in severe cases, coma.
Monitoring Requirements and Follow-Up
Recommended Monitoring Schedule
| Test / Visit | Timing | Estimated Cost | Purpose |
|---|---|---|---|
| Heartworm antigen test | Before starting; then annually | $35–$75 | Confirms dog is negative before prevention; detects breakthrough infection |
| Fecal flotation / parasite screen | Annually (often combined with HW test at wellness visit) | $25–$50 | Checks for intestinal parasites including whipworms not covered by Heartgard Plus |
| Weight check | Every 6–12 months | Included in wellness exam | Ensures correct dosage bracket — growing puppies or weight-changing adults may need a formulation change |
| MDR1 genetic test (if applicable) | Once, before starting (for at-risk breeds) | $65–$120 (WSU lab) | Identifies dogs with increased ivermectin sensitivity |
No routine blood chemistry panel is required specifically for Heartgard Plus monitoring in healthy dogs — the ivermectin dose is too low to cause hepatic or renal stress. However, your veterinarian may combine the heartworm test with a general wellness panel during annual exams.
Heartgard Plus Cost and Generic Availability
Heartgard Plus is one of the most widely prescribed canine medications, and pricing varies significantly depending on where you purchase it:
| Purchase Source | 6-Month Supply (26–50 lb dog) | 12-Month Supply (26–50 lb dog) | Notes |
|---|---|---|---|
| Veterinary clinic (direct) | $55–$80 | $95–$140 | Convenient; often includes auto-ship discounts |
| Verified online pharmacy (Chewy, 1-800-PetMeds) | $45–$70 | $80–$120 | Requires vet authorization; rebates available |
| Costco / Sam's Club pharmacy | $40–$60 | $75–$110 | Often lowest retail price; membership required |
Generic availability: As of 2026, several FDA-approved generic equivalents containing ivermectin and pyrantel pamoate at identical concentrations are available, including Iverhart Plus (Virbac), Tri-Heart Plus (Intervet/Merck), and Nuvermectin Plus. These generics typically cost 20–40% less than brand-name Heartgard Plus and carry the same safety and efficacy profile. Ask your veterinarian if a generic substitution is appropriate — in most cases, it is.
Manufacturer rebates: Boehringer Ingelheim frequently offers $10–$25 mail-in rebates on 6- and 12-month purchases of Heartgard Plus through veterinary clinics. Check their website or ask your vet's front desk about current promotions.
Alternatives to Heartgard Plus
If Heartgard Plus isn't the right fit for your dog — whether due to the need for broader parasite coverage, MDR1 concerns, or preference for a different administration route — several alternatives exist. Consult your veterinarian to determine the best option for your dog's specific risk profile.
| Product | Active Ingredients | Heartworm | Roundworms | Hookworms | Whipworms | Fleas/Ticks | Route |
|---|---|---|---|---|---|---|---|
| Heartgard Plus | Ivermectin + Pyrantel | ✅ | ✅ | ✅ | ❌ | ❌ | Oral chewable |
| Interceptor Plus | Milbemycin oxime + Praziquantel | ✅ | ✅ | ✅ | ✅ | ✅ (tapeworms) | Oral chewable |
| Simparica Trio | Sarolaner + Moxidectin + Pyrantel | ✅ | ✅ | ✅ | ❌ | ✅ | Oral chewable |
| Revolution Plus | Selamectin + Sarolaner | ✅ | ✅ | ✅ | ❌ | ✅ | Topical |
| ProHeart 6 / ProHeart 12 | Moxidectin (sustained-release) | ✅ | ❌ | ❌ | ❌ | ❌ | Injectable (vet-administered) |
When to consider switching: If your dog lives in an area with high whipworm prevalence (common in the southeastern U.S.), Interceptor Plus provides broader intestinal coverage. If you want flea and tick protection in a single product, Simparica Trio or Revolution Plus reduces the number of separate medications. If compliance with monthly dosing is a challenge, ProHeart 12 provides 12 months of heartworm prevention in a single veterinary injection — though it does not cover intestinal parasites and requires a separate deworming protocol.
Frequently Asked Questions
Can I give Heartgard Plus to my pregnant or nursing dog?
Yes. Heartgard Plus has been evaluated and is approved for use in breeding, pregnant, and lactating females. Studies showed no adverse effects on fertility, pregnancy outcomes, or nursing puppies when administered at the labeled dose. However, always confirm with your veterinarian before administering any medication during pregnancy.
My dog spit out the Heartgard Plus chewable. What should I do?
If you observed your dog spitting out the full chewable (not just crumbling), offer a replacement dose. If you're unsure whether any of the medication was ingested, contact your veterinarian for guidance. Do not double-dose without veterinary advice. To improve compliance, try breaking the chewable into smaller pieces and offering them one at a time as treats, or embed the chewable in a small amount of canned food.
Does Heartgard Plus protect against tapeworms?
No. Heartgard Plus does not contain praziquantel, the drug needed to treat tapeworms (Dipylidium caninum, Taenia species). If your dog has tapeworms — often visible as rice-like segments near the tail or in the stool — your veterinarian will prescribe a separate praziquantel treatment. Products like Interceptor Plus include tapeworm coverage.
Can cats take Heartgard Plus?
No. Heartgard Plus is formulated and approved exclusively for dogs. Cats require different heartworm preventives (such as Revolution or Heartgard for Cats, which contain different formulations and doses). Never administer canine Heartgard Plus to a cat, as the pyrantel dose and formulation are not appropriate for feline use.
Is it safe to buy Heartgard Plus from overseas online pharmacies?
The FDA and veterinary professionals strongly caution against purchasing medications from unverified international pharmacies. Products may contain incorrect active ingredient concentrations, be expired, or be counterfeit. Only purchase from Vet-Verified online pharmacies or your veterinarian's clinic. If a price seems too good to be true, the product's authenticity is likely questionable.
My dog weighs exactly 25 lb — which Heartgard Plus should I use?
Dogs at the exact boundary between two weight brackets (25 lb, 50 lb, or 100 lb) should be dosed at the higher bracket according to the product label. A 25 lb dog would receive the Brown package (26–50 lb). When in doubt, your veterinarian will make the final determination based on body condition score and weight trajectory.
Disclaimer: This article is for informational purposes only and does not replace professional veterinary advice. Always consult your veterinarian before starting, stopping, or changing any medication regimen for your dog. Heartgard Plus is a prescription medication that requires veterinary oversight.
Anouk Beaumont
All our authors care for dogs every day — read more of their work on the authors page.
