ProHeart 6 vs ProHeart 12 for Dogs: What Owners Should Know

Name: ProHeart 6 vs ProHeart 12 for Dogs: What Owners Should Know
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

ProHeart 12 for dogs is the longer-acting version of the same injectable heartworm preventive, given once a year instead of every six months. Both products use the same active ingredient, moxidectin, and both are given by a veterinarian as a subcutaneous injection. The main differences come down to duration, which dogs qualify, and how each fits into your dog's overall prevention plan.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

ProHeart 6 protects for 6 months per injection, and ProHeart 12 protects for 12 months per injection. Both are given in the clinic by a veterinarian.
Both contain moxidectin, a macrocyclic lactone that kills heartworm larvae before they grow into adult worms [1].
ProHeart 12 means one visit per year instead of two, which can improve compliance for busy owners.
Your dog must test negative for heartworm before either injection is given. Giving a preventive to a heartworm-positive dog can cause serious reactions [2].
Moxidectin has a wider margin of safety than ivermectin in dogs with the ABCB1 (formerly MDR1) gene defect, which appears in collies and some other breeds [1].

What Is ProHeart and What Do Veterinarians Use It For?

ProHeart is the brand name for an injectable heartworm preventive containing moxidectin. Moxidectin belongs to a drug class called macrocyclic lactones, which are the only medications currently licensed to prevent heartworm disease in dogs [1]. These drugs work by killing the third- and fourth-stage larvae of Dirofilaria immitis, the parasite that causes heartworm disease. If those larvae are killed early, adult worms never develop, and your dog never gets sick from heartworm [1].

Heartworm prevention matters because adult heartworms live in the pulmonary arteries and the right side of the heart. They can cause coughing, exercise intolerance, heart failure, and in severe cases, death. Treatment for adult heartworm infection exists, but it is expensive, takes months, and carries its own risks. Prevention is far easier and safer than treatment.

Veterinarians use ProHeart as a stand-alone heartworm preventive. It does not protect against fleas, ticks, or intestinal parasites, so many dogs also need a separate product for those. Your veterinarian will help you build a complete parasite control plan.

One practical advantage of the injectable form is that you do not have to remember a monthly pill. That matters because missed doses are one of the most common reasons heartworm prevention fails. A single injection removes that daily or monthly decision from your plate.

How Does Moxidectin Heartworm Prevention for Dogs Work?

Moxidectin is deposited under the skin and released slowly into the bloodstream over months. That slow release is what allows a single injection to protect your dog for a full season or longer.

Compared with ivermectin, another common macrocyclic lactone, moxidectin has a longer half-life and greater tissue distribution [1]. In plain terms, it stays in the body longer and reaches tissues where the larvae are developing. Laboratory studies have also shown that moxidectin is more effective than some other commonly used heartworm preventives against resistant strains of D. immitis [1]. That matters more each year, because heartworm resistance to older preventives has been documented in parts of the United States.

A field study in the Canary Islands, a region where heartworm is very common, followed dogs that received a single sustained-release moxidectin injection. The dogs lived outdoors in hunting kennels where heartworm prevalence ranged from 11.1% to 57.1%, with an average of 36.7% [3]. All 20 dogs that received the injection stayed heartworm-negative for the full study period and had no adverse reactions [3]. That is a strong signal that sustained-release moxidectin works even under heavy exposure pressure.

ProHeart 12 for Dogs: Dosing Basics

Who decides the dose?

Your veterinarian does. The dose is based on your dog's body weight and health status, and it is calculated at the clinic. Do not try to estimate a dose at home, and do not ask for a partial injection. Injectable heartworm preventives are prescription products given only by a licensed veterinarian.

How is it given?

It is a subcutaneous injection, which means it goes under the skin, usually between the shoulder blades or along the back. The visit is quick. Most dogs tolerate it well, and many owners report that their dog barely notices.

What if my dog misses a dose?

If your dog is late for a ProHeart 12 injection, call your veterinarian right away. The clinic will likely want to retest for heartworm before giving the next injection, because a gap in protection means your dog could have been exposed. The same applies to ProHeart 6. Do not wait until the next scheduled visit if you know you are overdue.

Does my dog need a heartworm test first?

Yes. A negative heartworm test is required before starting or continuing injectable prevention. This is standard practice for all heartworm preventives, and it is especially important with injectables because the protection lasts so long. If a dog is already infected, giving a preventive can trigger a reaction as microfilariae and adult worms die [2].

Side Effects: What to Watch For

Most dogs have no problems after a ProHeart injection. When reactions do occur, they are usually mild and short-lived. The table below covers the common and serious possibilities.

Side effect	How common or serious	What to do
Soreness, swelling, or a small lump at the injection site	Common and mild	Watch it for a few days. Call your vet if it grows, drains, or lasts more than a week.
Vomiting, diarrhea, or decreased appetite	Uncommon and usually mild	Offer water, monitor for 24 hours. Call your vet if it continues or your dog seems painful.
Lethargy or sleepiness	Uncommon and usually mild	Let your dog rest. Call your vet if it lasts more than a day or two.
Facial swelling, hives, or itching	Uncommon but a true allergic reaction	Call your veterinarian the same day.
Difficulty breathing, collapse, or seizures	Rare but an emergency	Go to a veterinary emergency hospital immediately.
Reaction in a heartworm-positive dog (fever, coughing, collapse)	Serious and preventable with testing	This is why testing before injection matters. Seek care right away if it happens [2].

Injectable moxidectin has been studied for safety in dogs, including dogs with pre-existing adult heartworm infections, where the main concern is a reaction to dying worms instead of the drug itself [2]. That is a good reminder that the pretest is not a formality.

If your dog has a heart condition, your veterinarian may listen for murmurs or arrhythmias before and after prevention visits. You can read more about what those sounds mean in Veterinary Stethoscopes for Pet Owners: Cardiac Acoustics, Bell vs. Diaphragm, and Murmur Monitoring.

Who Should Not Receive ProHeart, and What Interacts With It?

ProHeart is not right for every dog. Your veterinarian will review your dog's history before giving it.

Dogs that should not receive ProHeart include:

Dogs that test positive for heartworm. They need treatment first, not prevention.
Dogs with a known allergy to moxidectin or other macrocyclic lactones.
Dogs that are sick, debilitated, or recovering from surgery, until your veterinarian says they are stable.
Puppies below the minimum age listed on the product label. Your veterinarian will confirm whether your puppy is old enough.
Pregnant or breeding dogs, unless your veterinarian specifically recommends it after weighing the risks.

Drug interactions with moxidectin are not well established in veterinary research, but the class as a whole can interact with other drugs that affect the same nerve pathways. Tell your veterinarian about every medication, supplement, and parasite product your dog takes, including monthly chewables, flea and tick products, and anything you buy over the counter. If your dog takes a monthly preventive that also contains moxidectin, your veterinarian will make sure you are not doubling up.

One genetic consideration is the ABCB1 (formerly MDR1) gene defect, which is common in collies and some other herding breeds. Dogs with this defect can have severe reactions to ivermectin. Moxidectin has a greater margin of safety in these dogs compared with ivermectin [1]. That does not mean the defect should be ignored. It means your veterinarian can factor it into the decision.

Overdose and When to Call for Help

Injectable heartworm preventives are given by veterinarians at a calculated dose, so true overdose is uncommon. It can happen if a dog receives an injection meant for a much larger dog, or if a dog licks or eats a large amount of a moxidectin-containing chewable.

Signs of moxidectin toxicity can include drooling, vomiting, tremors, unsteady walking, dilated pupils, and in severe cases, seizures or coma. These signs can appear within hours.

If you think your dog received too much moxidectin, call your veterinarian or a pet poison control hotline right away. The ASPCA Animal Poison Control Center is available at (888) 426-4435, and the Pet Poison Helpline is available at (855) 764-7661. Both may charge a consultation fee. Have your dog's weight, the product name, and the amount given ready when you call.

Do not wait for symptoms to appear before calling. Early decontamination or supportive care is easier when it starts sooner.

Limitations and When to Contact a Veterinarian

This article covers general information. Your dog's individual case needs a veterinarian who can examine your dog, review the history, and run the appropriate tests.

Contact your veterinarian the same day if:

Your dog has facial swelling, hives, or intense itching after an injection.
Your dog vomits repeatedly or refuses food for more than 24 hours.
The injection site becomes hot, swollen, or painful.
You realize your dog is overdue for a ProHeart injection.

Go to an emergency hospital now if:

Your dog has trouble breathing, collapses, or has a seizure.
Your dog's gums look pale or blue.
Your dog cannot stand or seems disoriented.

Schedule a routine visit if:

You want to switch from a monthly preventive to an injectable.
You are not sure whether your dog is due for a heartworm test.
Your dog has a heart murmur and you want to discuss prevention options.

If your dog is due for a heartworm test and you are also thinking about gear changes for walks, the article on How To Choose A Harness For A Dog covers fit and comfort. A well-fitted harness makes clinic visits and exercise restriction easier if your dog ever needs heartworm treatment.

Frequently Asked Questions

Is ProHeart 12 safe for all dogs?

No, not for every dog. Dogs must test negative for heartworm first, and dogs with a known moxidectin allergy or certain health conditions should not receive it. Your veterinarian will decide whether your dog is a good candidate after an exam and a heartworm test.

Can I switch my dog from ProHeart 6 to ProHeart 12?

Yes, in most cases, if your dog tests negative for heartworm and your veterinarian approves. The switch usually happens at the time the next six-month injection would be due. Your veterinarian will set the timing based on your dog's history and exposure risk.

Does ProHeart protect against fleas and ticks?

No. ProHeart contains only moxidectin, which targets heartworm larvae. You will still need a separate flea and tick product if your dog is at risk. Your veterinarian can recommend one that works safely alongside the injection.

What happens if my dog is late for a ProHeart injection?

Call your veterinarian as soon as you realize it. A gap in protection means your dog may have been exposed to heartworm, so the clinic will likely retest before giving the next dose. Do not simply wait for the next scheduled appointment.

How do I know if my dog is at risk for heartworm?

Heartworm is transmitted by mosquitoes, so any dog that goes outside can be exposed, even in cooler states. Risk is higher in warm, humid regions and in dogs that spend a lot of time outdoors. Your veterinarian can assess your dog's specific risk and recommend the right prevention schedule.

References

A review of moxidectin vs. other macrocyclic lactones for prevention of heartworm disease in dogs with an appraisal of two commercial formulations. Frontiers in veterinary science, 2024.
Safety of Credelio Quattro™ (lotilaner, moxidectin, praziquantel, and pyrantel chewable tablets) in dogs infected with adult heartworms (Dirofilaria immitis). Parasites & vectors, 2025.
Efficacy of Sustained-Release Formulation of Moxidectin (Guardian SR) in Preventing Heartworm Infection over 18 Months in Dogs Living in a Hyperendemic Area. Animals : an open access journal from MDPI, 2024.