Heartgard Plus for dogs

DailyMed indications: prevent canine heartworm disease by eliminating the tissue stage of heartworm larvae (Dirofilaria immitis) for a month (30 days) after infection, and treat and control roundworms (Toxocara canis, Toxascaris leonina) and hookworms (Ancylostoma caninum, Uncinaria stenocephala, Ancylostoma braziliense).

CDC: D. immitis is transmitted by mosquitoes. In dogs, adults live in the pulmonary arteries (sometimes the right ventricle) and can live 5–10 years. Heartgard Plus does not kill those adults. The label says infected dogs must be treated to remove adult heartworms and microfilariae before starting a Heartgard Plus program, and that the product is not effective for microfilariae clearance.

Heartgard (ivermectin only) and Heartgard Plus share the same ivermectin dose (6 mcg/kg). Plus adds pyrantel. That is why Plus is labeled for those intestinal nematodes; it is still not a flea, tick, or whipworm product.

Label minimum dose: 6 mcg ivermectin per kg (2.72 mcg/lb) and 5 mg pyrantel per kg (as pamoate salt; 2.27 mg/lb), given orally at monthly intervals. Recommended for dogs 6 weeks of age and older. For dogs over 100 lb, use an appropriate combination of the chewables below. Do not invent a milligram dose from a blog chart—use the carton your veterinarian dispensed.

Administration: offer by hand or add intact to a small amount of food so the dog consumes the complete dose. Encourage chewing rather than swallowing whole—choking or intestinal obstruction has been reported. For dogs that gulp treats, the label says chewables may be broken into pieces. Watch for a few minutes; if part of the dose is lost, redosing is recommended. If you switch from another preventive, give the first Heartgard Plus dose within 30 days of the last dose of the former product. Missing more than a month can reduce ivermectin efficacy; the label says give it on or about the same day each month, and if delayed, treat immediately and resume the schedule.

Storage: 68–77°F (20–25°C); brief excursions 59–86°F permitted; protect from light. Keep out of reach of children and other animals. Human ingestion: contact a physician; Poison Control can advise. Cartons of 1, 6, or 12 chewables.

Label precaution: all dogs should be tested for existing heartworm infection before starting Heartgard Plus.

The American Heartworm Society’s June 2025 canine guidelines (summarized on AVMA as well): prescribe year-round FDA-approved preventives to prevent infection and improve compliance, especially because resistant heartworm subpopulations exist. AHS and FDA recommend year-round administration. AHS also recommends annual antigen and microfilaria testing for all dogs over 7 months, and antigen plus microfilaria tests before starting or changing products. Antigen is typically detectable about 5 months after infection; microfilariae about 6 months—so a “negative” too early can miss a prepatent infection.

Currently marketed FDA-approved preventives (ivermectin, milbemycin oxime, moxidectin, selamectin) are macrocyclic lactones. AHS: they are labeled for L3 and L4 larvae (and some products for microfilariae); oral and topical products are labeled on a 30-day interval. Heartgard Plus is one option—not the only FDA-approved preventive. AHS does not tell owners to skip mosquito control: reduce breeding sites and, when possible, outdoor time during peak feeding; FDA-approved ectoparasiticides and some EPA-registered mosquito repellents are discussed as add-ons, not as a replacement for a preventive.

Adult heartworm treatment is a separate veterinary protocol (AHS describes melarsomine regimens and does not recommend “slow-kill” with preventives alone). Do not start Heartgard Plus on a known-positive dog without your veterinarian’s plan.

Some herding-related dogs carry an ABCB1 (MDR1) mutation that weakens P-glycoprotein at the blood–brain barrier. AKC calls MDR1 a very important pharmacogene. The old internet rule “never give Heartgard to Collies” is not what the Heartgard Plus label says.

DailyMed safety: certain Collies are more sensitive to ivermectin at elevated doses (more than 16 times the target use level). At those high doses, reactions included mydriasis, depression, ataxia, tremors, drooling, paresis, recumbency, excitability, stupor, coma, and death. HEARTGARD showed no signs of toxicity at 10 times the recommended dose (60 mcg/kg) in sensitive Collies. The label concludes that those trials and bioequivalence studies support Heartgard products in dogs, including Collies, when used as recommended. It also reports a wide margin of safety at the recommended dose in pregnant or breeding bitches, stud dogs, and puppies 6 weeks and older.

Washington State University (which discovered the canine MDR1 mutation) lists Heartgard Plus on its MDR1-in-dogs drug table: the manufacturer tested it in MDR1 dogs; no adverse effects were observed; FDA determined Heartgard is safe for MDR1 dogs at label doses. Other ivermectin formulations are a different question—WSU says contact MDR1Caddie. WSU’s owner explainer: the heartworm-prevention dose of ivermectin (6 mcg/kg) is safe in dogs with the mutation; mange doses of 300–600 mcg/kg can cause neurologic toxicity in mutant/mutant dogs and can affect mutant/normal dogs. Livestock pour-ons and extra-label high doses are not the Heartgard Plus carton.

WSU approximate frequencies (their table; “unknown” means not enough data to quote a rate): Collie 70%; Australian Shepherd (and Mini Australian Shepherd) 50%; Chinook 25%; English Shepherd 15%; Shetland Sheepdog 15%; German Shepherd 10%; herding-breed cross 10%; mixed breed 5%; Old English Sheepdog 5%; Border Collie <5%; Golden Retriever and Siberian Husky unknown. Long-haired Whippet 50% and Silken Windhound 30% are on that list; they are not the same as a standard Whippet. Testing is how you know an individual dog’s status—WSU notes mixed-breed dogs can carry the mutation without looking like herding dogs. Dedicated MDR1 testing: dog DNA tests.

• Collie — energy 3/5
• Australian Shepherd — energy 5/5
• Shetland Sheepdog — energy 4/5
• Border Collie — energy 5/5
• German Shepherd — energy 5/5
• Old English Sheepdog — energy 3/5

Field trials on the label: vomiting or diarrhea within 24 hours of dosing was observed in 1.1% of administered doses. Post-approval experience (2022), listed by reporting frequency (not proven rates): vomiting, diarrhea, lethargy, anorexia, seizures, ataxia, muscle tremors, hypersalivation, pruritus. Choking or intestinal obstruction has been reported. FDA notes that not all events are reported, and frequency and causation cannot always be established from those reports. Adverse events: Boehringer Ingelheim 1-888-637-4251, or FDA 1-888-FDA-VETS / reportanimalae.

Heartgard Plus is not NexGard. NexGard (afoxolaner) is a flea-and-tick isoxazoline; WSU lists it separately and notes manufacturer MDR1 safety testing for that product. Combination products (for example those that add moxidectin) have their own labels. This page does not rank preventives. Ask your veterinarian which FDA-approved product matches parasites in your area, your dog’s other medications, and how reliably you can give a monthly chew.

Label: many flea collars, dips, shampoos, anthelmintics, antibiotics, vaccines, and steroids were used with Heartgard Plus in clinical trials—still tell your veterinarian every product you use. In one trial with parvovirus in some pups, efficacy against intestinal nematodes was marginally reduced, possibly from faster gut transit.

We do not publish clinic or Chewy stickers. Two national planning sources already in our cost library:

• AKC, “Are You Financially Prepared for a Puppy?”: $130–$180 for heartworm prevention and an annual test—a budget band, not a Heartgard Plus SKU.
• ASPCA 2021 “Cutting Pet Care Costs”: $185 a year for preventative medication as a generic national line—not brand-specific.

Actual cost depends on weight band, 6- vs 12-count cartons, and whether your clinic bundles an antigen test. Heartworm treatment after infection is a different, typically larger veterinary bill—AHS emphasizes prevention. Breed lifestyle still matters for mosquitoes and yard time; it does not change the label. Browse herding breeds or the match quiz if you are choosing a dog, then talk prevention with a veterinarian who examines that dog.