Heartgard Plus Chewables for Dogs

What Is Heartgard Plus Chewables for Dogs?

Heartgard Plus is a monthly oral preventive medication manufactured by Boehringer Ingelheim that protects dogs against heartworm disease (Dirofilaria immitis) and treats and controls intestinal parasites including roundworms (Toxocara canis, Toxascaris leonina) and hookworms (Ancylostoma caninum, Ancylostoma braziliense, Uncinaria stenocephala). The chewable comes in a beef-flavored, soft treat formulation that most dogs readily accept — making monthly dosing straightforward for owners.

Heartgard Plus contains two active ingredients at specific concentrations depending on the weight class of the product:

• Ivermectin: 68 mcg (Green — up to 25 lb), 136 mcg (Brown — 26–50 lb), 272 mcg (Blue — 51–100 lb)
• Pyrantel pamoate: 57 mg (Green), 114 mg (Brown), 227 mg (Blue)

According to the FDA's Center for Veterinary Medicine, Heartgard Plus was first approved for veterinary use in the United States in 1987, making it one of the longest-established heartworm preventives on the market. The medication is classified as an anthelmintic (anti-parasitic) combination product.

How Does Heartgard Plus Work in Dogs?

The combination is particularly effective because it addresses the two most common parasitic threats dogs face simultaneously: the life-threatening heartworm (transmitted by mosquitoes) and the zoonotic intestinal parasites roundworms and hookworms (which can transmit to humans, particularly children, through contaminated soil).

FDA-Approved Uses and Off-Label Applications

FDA-approved veterinary uses:

• Prevention of heartworm disease caused by Dirofilaria immitis in dogs and puppies 6 weeks of age and older
• Treatment and control of ascarids (roundworms): Toxocara canis and Toxascaris leonina
• Treatment and control of hookworms: Ancylostoma caninum, Ancylostoma braziliense, and Uncinaria stenocephala

Common off-label uses (under veterinary direction):

• Demodicosis (demodectic mange) management at higher doses — though higher-dose ivermectin protocols carry significant risk in susceptible breeds
• Sarcoptic mange adjunctive therapy in specific cases
• Some veterinarians use the heartworm-preventive dose as part of a broader parasite control protocol in multi-pet households

Off-label use always requires explicit veterinary authorization and monitoring. The dosages for mange treatment are substantially higher than heartworm prevention doses and carry increased risk of neurotoxicity, particularly in MDR1 gene mutation carriers.

Heartgard Plus Dosage by Dog Weight

The Merck Veterinary Manual recommends ivermectin at 6 mcg/kg (micrograms per kilogram) once monthly for heartworm prevention. Heartgard Plus is formulated to deliver this dose within specific weight brackets. Always dose based on your dog's current weight — not projected adult weight for growing puppies.

Important dosing notes: Dogs over 100 lb require multiple chewables combined to achieve appropriate dosing — your veterinarian will prescribe the correct combination. For puppies, start Heartgard Plus at 6 weeks of age or older and dose according to current weight, adjusting the product tier as the puppy grows. Puppies younger than 6 weeks should not receive this medication.

How to Administer Heartgard Plus

Heartgard Plus chewables are designed to be given orally once every 30 days. Here's what you need to know about proper administration:

• With or without food: Can be given on an empty stomach or with a meal. Some dogs with sensitive stomachs tolerate it better with food.
• Chewing vs. swallowing whole: The chewable should be chewed, not swallowed whole. For dogs that gulp treats, crumble the chewable and mix it into a small amount of food, or hold the dog's jaw gently closed after placing the chewable in the back of the mouth to encourage chewing.
• Timing: Give on the same date each month. Setting a calendar reminder or using the Heartgard Plus app reminder feature helps maintain the 30-day interval.
• Seasonal vs. year-round: The American Heartworm Society and the AVMA both recommend year-round heartworm prevention regardless of geographic location, as mosquitoes can survive indoors during winter and seasonal temperature patterns are increasingly unpredictable.
• Missed dose protocol: If you miss a dose, give the chewable immediately and resume the monthly schedule. If the gap exceeds 6 weeks, consult your veterinarian — a heartworm antigen test may be recommended before resuming.

Side Effects of Heartgard Plus

Heartgard Plus has a well-established safety profile after decades of clinical use. The ivermectin dose used for heartworm prevention (6 mcg/kg) is approximately 1/200th of the dose that causes toxicity in most dogs. However, adverse reactions can and do occur.

The MDR1 Gene Mutation: A Critical Safety Consideration

The MDR1 (multi-drug resistance 1) gene mutation affects the P-glycoprotein pump at the blood-brain barrier, allowing ivermectin and other drugs to cross into the central nervous system at doses that would be safe in dogs with the normal gene. Dogs homozygous for the MDR1 mutation (mutant/mutant) are significantly more sensitive to ivermectin toxicity.

Breeds with known MDR1 mutation prevalence:

• Collies: ~70% carry at least one copy of the mutation
• Australian Shepherds: ~50%
• German Shepherds: ~10–15%
• Shetland Sheepdogs: ~15%
• English Shepherds: ~15%
• Longhaired Whippets: ~10%
• Mixed breeds with herding ancestry: variable prevalence

At the heartworm prevention dose (6 mcg/kg), Heartgard Plus has been tested and shown to be safe even in MDR1 mutant dogs. However, the safety margin is narrower. If your dog is a herding breed or herding mix, discuss MDR1 genetic testing with your veterinarian before starting any ivermectin-containing product. Washington State University's Veterinary Clinical Pharmacology Laboratory offers the definitive MDR1 test.

Contraindications: Dogs Who Should NOT Receive Heartgard Plus

Your veterinarian will screen for these contraindications before prescribing:

• Heartworm-positive dogs: Dogs with adult heartworm infections should NOT start Heartgard Plus without veterinary management. Killing microfilariae rapidly can trigger anaphylactic reactions. Your vet will typically treat the adult infection first using melarsomine (Immiticide), then initiate prevention.
• Puppies under 6 weeks of age: Not approved for use in puppies younger than 6 weeks.
• Dogs with known hypersensitivity to ivermectin or pyrantel: Dogs that have had prior allergic reactions to either active ingredient should receive an alternative preventive class.
• Dogs with a positive heartworm antigen test: A negative antigen test is required before initiating Heartgard Plus. The test detects adult female heartworm antigen and typically costs $35–$65 at most veterinary clinics.
• Severely ill or debilitated dogs: Your veterinarian may delay preventive initiation until the dog's condition stabilizes, depending on the clinical scenario.

Drug Interactions with Heartgard Plus

Ivermectin is metabolized by the cytochrome P450 enzyme system (specifically CYP3A4) and is a substrate for the P-glycoprotein efflux transporter. Several commonly prescribed medications can increase ivermectin blood levels and raise the risk of neurotoxicity.

Always provide your veterinarian with a complete list of all medications, supplements, and over-the-counter products your dog is receiving before starting Heartgard Plus. This includes flea/tick preventives, joint supplements, and any herbal remedies.

Monitoring Requirements and Follow-Up

Heartgard Plus requires periodic veterinary monitoring to ensure ongoing safety and efficacy:

• Annual heartworm antigen test: Required every 12 months to confirm your dog remains heartworm-negative. Even with consistent monthly dosing, no preventive is 100% effective, and early detection of breakthrough infection dramatically improves treatment outcomes. Cost: $35–$65 per test.
• Annual wellness exam: Your veterinarian will assess overall health, update weight (which may change the Heartgard Plus tier needed), and review any new medications that could interact.
• Fecal parasite screening: Annual fecal flotation (cost: $25–$45) checks for parasites not covered by Heartgard Plus, including whipworms, tapeworms, Giardia, and Coccidia.
• Post-dose observation: For the first 2–3 doses, monitor your dog for 4–8 hours after administration for any signs of adverse reaction, particularly if your dog is a herding breed.

Cost of Heartgard Plus and Generic Availability

Heartgard Plus is available by prescription from your veterinarian, through veterinary online pharmacies (with vet authorization), and at major pet pharmacy retailers. Pricing varies based on dog weight tier and supply duration.

Generic equivalents contain the same active ingredients at the same concentrations and are FDA-approved as bioequivalent. Iverhart Plus (Virbac) and Tri-Heart Plus (Virbac/other manufacturers) are the most widely available generics and typically cost 15–30% less than brand-name Heartgard Plus. NuHeart (formerly Heartgard Plus generic by IVS) is another option available through some online pharmacies.

Manufacturer rebate programs: Boehringer Ingelheim periodically offers rebates on 12-month purchases — check their website or ask your veterinary clinic about current promotions. Purchasing a 12-month supply at once is almost always more cost-effective than buying 6-month supplies twice.

What to Do If You Miss a Dose or Overdose Occurs

Missed dose:

1. Give the missed chewable as soon as you remember.
2. Reset your monthly calendar from the date of the late dose (not the originally scheduled date).
3. If the gap was less than 6 weeks, the missed larval stage should still be covered by the retroactive action of the dose you just gave.
4. If the gap exceeded 6 weeks, contact your veterinarian. They may recommend a heartworm antigen test before or shortly after resuming prevention.
5. If the gap exceeded 6 months, a heartworm test is strongly recommended before restarting, as adult worms may have developed.

Overdose (dog consumed multiple chewables):

1. Estimate how many chewables were consumed and the total ivermectin dose ingested (check the mcg per chewable on the package).
2. Call your veterinarian or an emergency veterinary clinic immediately.
3. For guidance outside business hours, contact ASPCA Animal Poison Control: (888) 426-4435 or Pet Poison Helpline: (855) 764-7661.
4. Do NOT induce vomiting unless instructed by a veterinary professional.
5. Monitor for signs of ivermectin toxicity: dilated pupils, disorientation, tremors, drooling, vomiting, blindness, seizures, or coma.
6. Toxicity signs typically appear within 4–12 hours of ingestion but can be delayed up to 24 hours.

At the heartworm prevention dose, the margin of safety for ivermectin is approximately 10x in normal dogs and 1.6x in MDR1-sensitive dogs. Consuming 2–3 chewables of the correct weight-tier product is unlikely to cause severe toxicity in a normal dog but warrants veterinary consultation. Consuming an entire box or chewables from a higher weight tier is a genuine emergency.

Alternatives to Heartgard Plus

Several alternative heartworm preventives are available if Heartgard Plus isn't the right fit for your dog. Your veterinarian will consider your dog's breed, weight, health status, parasite risk, and your lifestyle preferences when recommending an alternative.

For dogs with confirmed MDR1 mutations or prior adverse reactions to ivermectin, Interceptor Plus (milbemycin oxime) or ProHeart (moxidectin) are common alternatives. However, milbemycin and moxidectin are also macrocyclic lactones, so the decision should be made with your veterinarian based on the specific mutation status and severity of prior reactions.

Frequently Asked Questions

Can I give Heartgard Plus to my cat?

No. Heartgard Plus is formulated and dosed specifically for dogs. The ivermectin and pyrantel concentrations are not appropriate for cats. Boehringer Ingelheim does not produce a feline version of this product. Consult your veterinarian for cat-appropriate heartworm preventives such as Revolution (selamectin) or Heartgard for Cats.

Does Heartgard Plus protect against fleas and ticks?

No. Heartgard Plus only covers heartworms, roundworms, and hookworms. You will need a separate flea and tick preventive if you live in an area where these parasites are prevalent. Products like Simparica Trio combine heartworm prevention with flea and tick coverage in a single monthly chewable.

How long does my dog need to be on Heartgard Plus?

Heartworm prevention is a lifelong commitment for most dogs. The American Heartworm Society recommends year-round prevention for all dogs regardless of geographic location. Stopping prevention leaves your dog vulnerable to heartworm infection, which requires expensive treatment ($1,000–$3,000+) and carries significant health risks. Continue Heartgard Plus for your dog's entire life unless your veterinarian recommends switching to an alternative product.

What if my dog vomits after taking Heartgard Plus?

If your dog vomits within 1 hour of administration, the dose may not have been fully absorbed. Contact your veterinarian — they may recommend re-dosing. If vomiting occurs more than 2 hours after administration, the medication was likely absorbed. Recurrent vomiting after dosing should be reported to your vet, who may recommend switching to a topical or injectable alternative.

Is Heartgard Plus safe for pregnant or nursing dogs?

Yes. Heartgard Plus has been evaluated for safety in breeding, pregnant, and lactating dogs and is approved for use in these populations. However, always confirm with your veterinarian before administering any medication to a pregnant or nursing dog, as individual health factors may influence the recommendation.

Can I buy Heartgard Plus without a prescription?

No. Heartgard Plus is a prescription medication that legally requires veterinary authorization. Online pharmacies that sell it without a prescription are operating outside FDA regulations and may sell counterfeit, expired, or improperly stored product. Always obtain Heartgard Plus through your veterinarian or a verified veterinary pharmacy that requires prescription authorization.