Tri-Heart Plus is a well-regarded, FDA-licensed heartworm preventative that uses the same active ingredients as the better-known Heartgard Plus, typically at a lower price. It’s made by Merck Animal Health and protects against heartworm disease, two species of roundworm, and three species of hookworm. For most dog owners looking for reliable, affordable monthly heartworm prevention, it’s a solid choice.
What Tri-Heart Plus Protects Against
Tri-Heart Plus is a monthly chewable tablet that prevents heartworm disease by killing heartworm larvae during the tissue stage, before they can mature into adults that damage the heart and lungs. It works retroactively: each dose eliminates any larvae your dog picked up from mosquito bites during the previous 30 days.
Beyond heartworm, it also treats and controls intestinal parasites. Specifically, it targets two roundworm species (Toxocara canis and Toxascaris leonina) and three hookworm species (Ancylostoma caninum, Uncinaria stenocephala, and Ancylostoma braziliense). This combination means your dog gets both heartworm prevention and a monthly intestinal deworming in one tablet.
What it does not cover: Tri-Heart Plus offers no protection against fleas, ticks, whipworms, or tapeworms. If your dog needs broader parasite coverage, you’d pair it with a separate flea and tick product or consider an all-in-one alternative.
How It Compares to Heartgard Plus
This is the question most people are really asking. Tri-Heart Plus and Heartgard Plus contain identical active ingredients: ivermectin and pyrantel pamoate, at the same concentrations. They target the same parasites at the same doses. Both are FDA-licensed, meaning both had to demonstrate safety and efficacy to earn approval. The products are grouped together on veterinary comparison charts for this reason.
The main practical differences come down to flavor, texture, and cost. Tri-Heart Plus tends to be noticeably cheaper per dose, which adds up over a dog’s lifetime of monthly prevention. Some dogs have a preference for one chewable over the other based on taste or consistency, but both are flavored tablets designed to be given as treats. If your dog happily eats one, there’s no medical reason to switch to the other.
Dosing by Weight
Tri-Heart Plus comes in three tablet strengths based on your dog’s weight:
- Up to 25 lbs: 68 mcg ivermectin, 57 mg pyrantel
- 26 to 50 lbs: 136 mcg ivermectin, 114 mg pyrantel
- 51 to 100 lbs: 272 mcg ivermectin, 227 mg pyrantel
Dogs over 100 pounds get a combination of tablets to reach the correct dose. Each pack contains six chewable tablets, covering six months of protection. The tablets can be broken into pieces for dogs that tend to swallow treats whole, but watch your dog for a few minutes afterward to make sure the entire dose is consumed. If any portion is lost or spit out, give another full dose.
Timing matters. Doses should be given at consistent monthly intervals. Missing a month or giving doses late creates a window where larvae can mature past the stage that ivermectin kills, potentially leading to a heartworm infection even in a dog that’s “on” prevention.
Safety and Side Effects
At the low doses used for heartworm prevention, ivermectin-based chewables have a strong safety track record. The most commonly reported side effects across ivermectin/pyrantel products are vomiting, lethargy, diarrhea, and decreased appetite, typically appearing within a day of dosing and resolving on their own. Serious reactions like hypersensitivity are rare, occurring in roughly 2% of dogs in comparative studies of heartworm preventatives.
One concern that comes up frequently is ivermectin sensitivity in herding breeds. Collies, Shetland Sheepdogs, Australian Shepherds, Old English Sheepdogs, and long-haired Whippets can carry a gene mutation (MDR1) that makes them vulnerable to ivermectin toxicity. However, this is a dose-dependent issue. The amount of ivermectin in heartworm preventatives like Tri-Heart Plus is far below the threshold that causes problems, even in MDR1-positive dogs. The risk surfaces with the much higher doses used off-label for skin parasites like mange, not with standard monthly prevention. Genetic testing is available if you want confirmation for your specific dog, but veterinary sources consistently state that FDA-approved heartworm preventive doses are safe for these breeds.
Testing Before and During Use
Your dog needs a heartworm test before starting Tri-Heart Plus. Giving a preventative to a dog that already has an active heartworm infection can cause dangerous complications. The American Heartworm Society recommends annual testing that includes both an antigen test (which detects proteins from adult female heartworms) and a microfilaria test (which checks for circulating baby heartworms in the blood). The microfilaria test is especially important when a dog’s prevention history is unknown or infection is suspected.
Annual testing remains important even for dogs on consistent prevention. No preventative is 100% foolproof, particularly if a dose was given late, partially consumed, or vomited up without the owner noticing.
Storage and Shelf Life
Store Tri-Heart Plus at room temperature between 59 and 86°F and keep it out of direct light. Don’t leave it in a hot car or a sunny windowsill. The tablets come in individually sealed packaging, so keep them sealed until you’re ready to give a dose.