Protecting your dog from heartworm disease is one of the most important responsibilities of pet ownership. Tri-Heart Plus (ivermectin/pyrantel) chewable tablets offer a reliable, veterinarian-recommended solution — not only preventing heartworm disease but also treating and controlling common intestinal parasites. Whether you’re a first-time dog owner or a seasoned pet parent, understanding how this medication works, how to use it correctly, and what precautions to take can make all the difference in your dog’s long-term health.
What Is Tri-Heart Plus?
Tri-Heart Plus is a combination oral antiparasitic medication containing two active ingredients: ivermectin and pyrantel pamoate. It is available exclusively by prescription and is restricted to use by or on the order of a licensed veterinarian.
The product is indicated for:
- Prevention of canine heartworm disease by eliminating the tissue stage of Dirofilaria immitis larvae within 30 days after infection
- Treatment and control of roundworms: Toxocara canis and Toxascaris leonina
- Treatment and control of hookworms: Ancylostoma caninum, Uncinaria stenocephala, and Ancylostoma braziliense
Dosage and Administration
Tri-Heart Plus is administered orally once a month at a minimum dose of 6 mcg of ivermectin per kilogram and 5 mg of pyrantel (as pamoate salt) per kilogram of body weight. The dosing schedule based on your dog’s weight is as follows:
| Dog Weight | Tablets Per Month | Ivermectin | Pyrantel | Color Code |
|---|---|---|---|---|
| Up to 25 lbs | 1 | 68 mcg | 57 mg | Blue |
| 26–50 lbs | 1 | 136 mcg | 114 mg | Green |
| 51–100 lbs | 1 | 272 mcg | 227 mg | Brown |
For dogs over 100 lbs, use the appropriate combination of tablets. Tri-Heart Plus is suitable for dogs 6 weeks of age and older.
How to Give the Tablet
Remove only one chewable tablet at a time from the blister card. Because most dogs find the product palatable, it can be offered directly by hand. It may also be added to a small amount of dry food or placed at the back of the dog’s mouth for forced swallowing.
Always ensure your dog has consumed the complete dose. Observe your dog for a few minutes after administration. If you suspect part of the dose was lost or rejected, re-dosing is recommended. For dogs that tend to swallow treats whole, consider breaking the chewable into smaller pieces to prevent the risk of choking or intestinal obstruction.
Timing and Consistency Matter
Tri-Heart Plus should be given at monthly intervals during the period when mosquitoes — the vectors that carry infective heartworm larvae — are active. Key timing guidelines include:
- The first dose must be given within 30 days of the dog’s first mosquito exposure.
- The final dose must be given within 30 days of the last mosquito exposure.
- When switching from another heartworm preventive, the first Tri-Heart Plus dose must be given within 30 days of the last dose of the previous medication.
If the dosing interval exceeds 30 days, ivermectin’s efficacy may be reduced. To maintain optimal protection, administer the tablet on or about the same day each month. If treatment is delayed — even by just a few days — give the dose immediately and resume the regular monthly schedule to minimize the risk of adult heartworm development.
Monthly use also provides ongoing treatment and control of roundworms and hookworms. Pet owners should also take measures to prevent reinfection with intestinal parasites, such as maintaining clean living environments and practicing good hygiene.
Efficacy
Clinical studies confirm that Tri-Heart Plus chewable tablets, when administered at the recommended dose and regimen, are highly effective:
- Against heartworm larvae (D. immitis): Eliminates the tissue larval stage within 30 days post-infection, preventing adult heartworm development.
- Against roundworms: Effective against T. canis and T. leonina.
- Against hookworms: Effective against A. caninum, U. stenocephala, and A. braziliense.
In acceptability trials, the majority of dogs consumed Tri-Heart Plus at first offering, confirming its palatability as a chewable tablet.
Important Precautions Before Starting Treatment
All dogs must be tested for existing heartworm infection before beginning Tri-Heart Plus. This is because the medication is not effective against adult D. immitis. Dogs already infected with heartworms must first be treated to eliminate adult heartworms and microfilariae before starting a preventive program.
While ivermectin at the recommended dose may kill some microfilariae, Tri-Heart Plus is not intended or approved for microfilariae clearance. In some clinical trials, a mild hypersensitivity-type reaction — particularly transient diarrhea — was observed in dogs with circulating microfilariae after treatment with ivermectin alone.
Additional precautions include:
- Keep out of reach of children. In case of human ingestion, contact a physician or Poison Control Center immediately.
- Store securely away from dogs, cats, and other animals to prevent accidental ingestion or overdose.
- Storage conditions: Store at controlled room temperature 59–86°F (15–30°C). Protect from light.
Adverse Reactions and Post-Approval Experience
In clinical field trials, vomiting or diarrhea within 24 hours of dosing was observed in approximately 1.1% of administered doses.
Based on post-approval adverse drug experience reporting, the following adverse events have been documented in decreasing order of frequency:
- Vomiting
- Diarrhea
- Lethargy
- Anorexia
- Seizures
- Ataxia
- Muscle tremors
- Hypersalivation
- Pruritus (itching)
Additionally, choking or intestinal obstruction has been reported in some cases following administration. As noted above, breaking the tablet into pieces for dogs that swallow treats whole can reduce this risk.
Safety Profile
Extensive research supports the safety of Tri-Heart Plus when used as directed:
- Collie breeds and related dogs: Some Collie-breed dogs are more sensitive to elevated doses of ivermectin (more than 16 times the target dose). At such elevated levels, sensitive dogs showed signs including mydriasis, ataxia, tremors, depression, paresis, coma, and death. However, no signs of toxicity were observed at 10 times the recommended dose (60 mcg/kg) even in sensitive Collies, confirming that the product is safe at recommended levels.
- Broad safety margin: Tri-Heart Plus has demonstrated a wide margin of safety at the recommended dose in dogs, including pregnant or breeding females, stud dogs, and puppies 6 weeks and older.
- Compatibility: In clinical trials, the medication was safely used alongside many common flea collars, dips, shampoos, anthelmintics, antibiotics, vaccines, and steroid preparations.
- Note on parvovirus: In one trial, puppies infected with parvovirus showed a marginal reduction in efficacy against intestinal nematodes, possibly due to changes in intestinal transit time.
How Tri-Heart Plus Is Supplied
Tri-Heart Plus is available in three dosage strengths corresponding to the three dog weight ranges. Each strength is packaged in convenient packs of 6 chewable tablets.
Manufactured for: Intervet Inc., a subsidiary of Merck & Co., Inc., 126 E. Lincoln Avenue, Rahway, NJ 07065
Manufactured by: Diamond Animal Health, Inc., Des Moines, IA 50327
For more information or to report suspected adverse drug experiences, contact Merck Animal Health at 1-800-224-5318 or visit www.TriHeartPlus.com. To report adverse drug experiences to the FDA, contact 1-888-FDA-VETS or visit http://www.fda.gov/reportanimalae.
Approved by FDA under ANADA #200-338.
Final Thoughts
Tri-Heart Plus is a trusted, FDA-approved heartworm preventive that also addresses two of the most common intestinal parasites affecting dogs. With consistent monthly use, proper timing, and a pre-treatment heartworm test, it forms a cornerstone of a responsible canine health program. Always consult your licensed veterinarian before starting or changing your dog’s parasite prevention plan to ensure the safest and most effective approach for your pet’s individual needs.
References:
- Tri-Heart Plus Package Insert, Intervet Inc. / Merck Animal Health, ANADA #200-338
- U.S. Food & Drug Administration – Center for Veterinary Medicine: http://www.fda.gov/reportanimalae
- Merck Animal Health Contact: 1-800-224-5318 | www.TriHeartPlus.com