Antiparasitic combination · For Dogs

Heartgard Plus dosage reference for dogs (Ivermectin / Pyrantel)

This product is presented with its label's weight-band table instead of a weight-based calculator. Confirm the current weight, exact product strength and schedule with your veterinarian.

Onset: N/A (preventive)
Duration: 30 days
Class: Antiparasitic combination

This information is for educational purposes only. Always consult your veterinarian before giving medication to your pet. Do not use this reference for emergencies, overdoses, pregnancy, very young animals, very old animals, pets with liver/kidney disease, or pets taking multiple medications.

Sources and review process

Each medication page lists the references recorded for its data. The reference content and editorial presentation are maintained by PetDosageChart; a page is independently DVM-reviewed only when a named reviewer byline appears. See our methodology and limitations.

Heartgard Plus is a prescription combination of ivermectin and pyrantel for heartworm prevention and the treatment and control of specified roundworms and hookworms. The US label supplies fixed chew strengths by dog-weight range. A calculation using ivermectin alone does not describe the complete combination chew.

Heartgard Plus Weight Ranges and Chew Strengths

The US label lists the following monthly strengths for dogs at least 6 weeks old. Ivermectin is listed in micrograms (mcg) and pyrantel in milligrams (mg). These units are different. Confirm the current weight and prescribed box strength with your veterinarian.

US label: dogs at least 6 weeks old. Ranges are reproduced as printed. If an exact weight falls between the printed ranges, ask your veterinarian to confirm the strength. The label permits breaking the prescribed chew into pieces for administration only if the complete dose is consumed at one time.

Dog weight	Label dose (once monthly)
Up to 25 lb	One chewable: 68 mcg ivermectin / 57 mg pyrantel
26 to 50 lb	One chewable: 136 mcg ivermectin / 114 mg pyrantel
51 to 100 lb	One chewable: 272 mcg ivermectin / 227 mg pyrantel
Over 100 lb	An appropriate combination of chewables selected by the prescribing veterinarian

Source: Heartgard Plus US prescribing label (DailyMed) — Dosage table; Administration; Safety.

Use the complete dose prescribed for the labeled weight band. Do not divide a larger chew to create a smaller dose or combine strengths without your veterinarian's instructions. Confirm the product and schedule with your veterinarian.

Product check: This is the US Heartgard Plus label, not standalone ivermectin or Heartgard for cats. Do not divide a larger chew to create a smaller dose. Dogs over 100 lb need a veterinarian-selected combination of chewables.

By Weight

Heartgard Plus reference by dog weight

Choose a common weight to open its optional reference page. Return to the main guide for the product's labeled weight-band limits.

Common-weight pages are convenience views of the same educational reference, not separate medical recommendations.

How to Give Heartgard Plus to Your Dog

Follow the prescribing clinic’s monthly schedule and heartworm-testing instructions. Ensure the dog consumes the complete prescribed dose. The label says the chew should be chewed rather than swallowed whole; for dogs that swallow treats whole, it permits breaking the prescribed chew into pieces and feeding the complete dose at one time. This is different from saving part of a larger chew as a smaller dose.

It is generally recommended to give Ivermectin / Pyrantel with food to help reduce stomach upset. It may interact with spinosad (may increase ivermectin levels) and P-glycoprotein inhibitors.

Side Effects and Safety

Common side effects include vomiting, diarrhea, decreased appetite, and lethargy. More serious side effects can occur, such as allergic reaction, tremors (rare), and seizures (rare, MDR1 breeds at higher doses).

Cautions · 12

When Heartgard Plus should be used with caution or avoided

Some dogs should not take Heartgard Plus, or need a different dose and closer monitoring. Tell your veterinarian about your pet's full health history and every other medication or supplement they take before giving Heartgard Plus.

dogs not tested for heartworm
known ivermectin sensitivity (MDR1 mutation at higher doses)

Interactions · 13

Heartgard Plus drug interactions

Heartgard Plus can interact with other drugs. This list is not complete — always give your veterinarian a full list of your dog's medications and supplements.

spinosad (may increase ivermectin levels)

P-glycoprotein inhibitors

Reference Limits · 14

When this reference is not enough

A reference table cannot decide whether a product or treatment is appropriate for an individual animal. Do not rely on it in these situations — talk to your veterinarian instead:

In an emergency, or after accidental ingestion or a suspected overdose
For pregnant or nursing pets
For very young or very old dogs
For pets with liver disease, kidney disease, or another chronic illness
For pets taking several medications at once
When you are not sure of your dog's current, accurate weight
For combination products that contain additional active ingredients

Overdose · 15

Suspected overdose: what to do

If you think your dog had too much Heartgard Plus, or swallowed any medication by accident, act right away. Do not wait for symptoms to appear.

Call your veterinarian or the nearest emergency animal hospital.
ASPCA Animal Poison Control Center: (888) 426-4435
Pet Poison Helpline: (855) 764-7661

A consultation fee may apply to the poison hotlines. Have the medication packaging ready, and note how much your pet may have taken and when. Do not try to make your pet vomit unless a veterinarian tells you to.

When to Call the Vet

If your dog experiences any of the following, contact your veterinarian or an emergency clinic immediately:

Vomiting within 1 hour of dosing
Your dog misses more than one monthly dose
You moved from a non-endemic to an endemic heartworm area
Any neurological signs (tremors, incoordination)
Your dog tests positive for heartworm

Frequently Asked Questions

Frequently Asked · 07

Questions about this medication

Is Heartgard enough for my dog?

Heartgard Plus prevents heartworm and treats roundworms and hookworms, but it does NOT protect against fleas, ticks, or other external parasites. Most dogs need a flea/tick preventive (like NexGard) in addition to Heartgard. Your vet can recommend a complete parasite prevention plan.

What happens if I miss a month of Heartgard?

If you miss one month, give the dose as soon as you remember and resume the monthly schedule. If you miss more than one month, contact your vet — your dog may need to be tested for heartworm before restarting, as the preventive only kills larvae acquired in the previous 30 days.

Can I give Heartgard to Collies?

Yes. At the very low ivermectin doses used in Heartgard (6 mcg/kg), it is considered safe for Collies and other MDR1-positive breeds. The ivermectin toxicity risk in these breeds occurs at much higher doses (300+ mcg/kg), far above what Heartgard contains.

Does my dog need Heartgard year-round?

The American Heartworm Society recommends year-round heartworm prevention in all areas. Even in colder climates, unpredictable warm spells can allow mosquitoes to transmit heartworm. Year-round prevention is the safest approach.

Related · 08

Related medications for dogs

These references share a recorded drug class. They are not interchangeable treatments or instructions to combine medications.

Sources · 09

Sources & references

The structured medication record behind this page names the references below. The links open the publisher or official resource; they do not turn this page into a prescription for an individual animal.

Heartgard Plus US prescribing label (DailyMed)
Dosage table; Administration; Safety. Supports the specific claim described above; it is not a review of the entire page.
Plumb's Veterinary Drug Handbook — listed in this medication's source record.
Merck Veterinary Manual — listed in this medication's source record.
DailyMed label search for Ivermectin / Pyrantel — check current product and formulation labels; some veterinary uses may be off-label.

Specific citations above support the identified claims. Other source names remain record-level references; further claim review is still needed. Read our methodology and limitations.

Transparency · 11

How this page was made

This page's structured medication record includes Plumb's Veterinary Drug Handbook among its source names. This product is presented with its label's weight-band table instead of a weight-based calculator. A source record is not the same as an individualized prescription or an independent clinical review.

Sources cited: Plumb's Veterinary Drug Handbook
Merck Veterinary Manual
Calculator formula: No weight-based calculator is shown because this product is dosed by the weight bands on its label: one whole labeled product covers every weight in a band. Use the exact labeled strength directed by the veterinarian.
Editorial owner: Prepared and maintained by the PetDosageChart editorial team, a software-led publisher and not a veterinary practice.
DVM review status: Not independently DVM-reviewed. A named reviewer and review date appear here only after a confirmed review.

More on our methodology and editorial process — how we source, calculate, and review dogs dosage data.

Safety · 10

More resources

Pet Medication Safety Hub — What's safe, what's dangerous, and emergency signs
How we calculate dosages — Our sources, math, and review process
Printable emergency checklist — One-page reference for your fridge or first aid kit