ProHeart Injection Side Effects in Dogs: What to Watch For

Name: ProHeart Injection Side Effects in Dogs: What to Watch For
Creator: Dr. Zubair Khalid

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

Most dogs get a ProHeart injection with no problems beyond a sore spot at the shot site. When reactions do happen, the most common signs are vomiting, diarrhea, tiredness, or swelling where the injection was given. Serious reactions are rare, but they can happen, and knowing what to watch for helps you act fast. This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

ProHeart is a long-acting heartworm prevention injection. Your veterinarian decides the dose based on your dog's weight and health.
Common signs include vomiting, diarrhea, low energy, and tenderness or swelling at the injection site.
Serious reactions are uncommon but can include severe allergic responses, neurologic signs, or liver problems.
Call your veterinarian the same day if your dog vomits repeatedly, seems very tired, or has swelling at the shot site that is getting worse.
Go to an emergency clinic right away for collapse, trouble breathing, seizures, or a swollen face.

What Is ProHeart and What Is It Used For?

ProHeart is an injectable heartworm preventive. The active ingredient is moxidectin, a drug in the macrocyclic lactone family. Macrocyclic lactones are the same broad group that includes the monthly heartworm preventives many owners already use in chewable or topical form. The difference with ProHeart is that it is given as a shot in the clinic and releases slowly over time, so you do not have to remember a monthly dose.

Veterinarians use it to prevent heartworm disease caused by Dirofilaria immitis. Heartworm disease can lead to chronic heart problems and lung damage, and treating it once it develops is long and costly [1]. Prevention is far easier than treatment. Moxidectin also shows up in heartworm treatment protocols, where it is combined with other drugs to kill young worms and reduce transmission [2][3].

ProHeart is a preventive, not a treatment for adult heartworms. If your dog already has adult heartworms, your veterinarian will build a different plan. That plan may include a sustained-release moxidectin product along with doxycycline, prednisone, and melarsomine injections [2]. If your dog is on prednisone as part of that plan, it helps to know what that drug can do on its own. You can read more about prednisone side effects in dogs here: What Are the Side Effects of Prednisone in Dogs?.

How Does the Injection Work?

The moxidectin in ProHeart is formulated to be absorbed slowly from the injection site. Instead of a single burst of drug, your dog gets a steady low level of medication over months. That steady level is what kills heartworm larvae (the immature stages) that mosquitoes deposit after a bite.

Because the drug stays in the body for a long time, a reaction that starts soon after the shot can sometimes last longer than a reaction to a pill. This is one reason your veterinarian will ask about your dog's history before giving the injection. It is also why the injection is not the right choice for every dog.

Dosing Basics

Your veterinarian decides the dose. The amount depends on your dog's body weight and overall health, and the clinic calculates it for your specific dog. There is no at-home version of this injection, and you should never try to adjust or repeat a dose on your own.

The injection is given by a veterinarian or trained veterinary staff member, usually in the clinic. Your dog does not need to be sedated for a routine preventive injection. After the shot, most clinics will ask you to wait a short time in the lobby so staff can watch for an immediate reaction.

If your dog misses a scheduled booster, call your veterinarian. Do not wait and hope the protection still covers your dog. The clinic will tell you whether a new injection is needed and whether a heartworm test should be repeated first. Timing matters because the gap between doses affects how well the drug keeps working.

ProHeart for Dogs Side Effects: What Owners Should Know

Most reactions are mild and short. The table below covers the signs owners report most often, plus the ones that need urgent care.

Side effect	How common or serious	What to do
Vomiting	Common, usually mild	Withhold food for a few hours, offer small amounts of water. Call your vet if it continues past a day or comes with other signs.
Diarrhea	Common, usually mild	Keep water available. Call your vet if diarrhea is bloody, severe, or lasts more than a day or two.
Tiredness or low energy	Common, usually mild	Let your dog rest. Call your vet if your dog will not get up, seems weak, or is not interested in food.
Swelling, tenderness, or a lump at the injection site	Common, usually mild	Watch the spot. Call your vet if it grows, feels hot, or your dog cries when it is touched.
Itching, hives, or a swollen face	Uncommon, serious	This can be an allergic reaction. Call your vet right away or go to an emergency clinic.
Trouble breathing, collapse, or seizures	Rare, emergency	Go to an emergency clinic immediately.
Yellow gums, yellow eyes, or dark urine	Rare, serious	These can signal liver problems. Call your vet the same day.
Stiffness, wobbling, or trouble walking	Rare, serious	Stop activity and call your vet right away.

Some of these signs overlap with reactions to other preventive products. If your dog has had trouble with a chewable preventive before, tell your veterinarian. Reactions to other flea and tick or heartworm products can look similar, and that history helps your vet choose the safest option. For comparison, here is what owners often see with another common preventive: What Are the Side Effects of Bravecto for Dogs? What to Watch For.

Common Reactions

Vomiting, diarrhea, and a day or two of low energy are the signs owners notice most. A sore or slightly swollen spot where the needle went in is also common. These usually fade on their own with rest and bland food.

Serious Reactions

Serious reactions are rare, but they are the reason clinics ask you to wait after the injection. Severe allergic reactions can start within minutes. Neurologic signs, such as wobbling or trouble walking, and liver problems are reported far less often but are more concerning when they appear. If you see any of the emergency signs in the table, do not wait to see if things improve.

Who Should Not Get ProHeart?

Tell your veterinarian about your dog's full history before the injection. Dogs with a known allergy to moxidectin or to other drugs in the same family should not receive it. Dogs who have had a reaction to a previous dose should not get another one without a careful discussion.

Your veterinarian will also weigh the risks for dogs with liver disease, dogs who are very young or very old, and dogs who are already weak or ill. Heartworm-positive dogs need a different plan, because prevention and treatment are not the same thing. In heartworm treatment, moxidectin is used alongside other drugs under close veterinary supervision, and the protocol is built around the dog's disease class [2][3].

Drug Interactions

Some medications and health conditions can change how your dog handles moxidectin. The most important step is a complete medication list at your visit. Include every preventive, supplement, and prescription your dog takes, even ones you buy without a prescription.

If your dog takes anxiety medication, for example, your veterinarian will want to know. Some of those drugs can cause drowsiness or coordination problems on their own, and that can make it harder to tell what is causing a new sign. You can read about those effects here: Dog Anxiety Medication Side Effects: What to Watch For.

Overdose and When to Call for Help

An overdose with this product is unlikely at home, because the injection is given in the clinic. If you think your dog received too much, or if your dog has a severe reaction, call your veterinarian first. If the clinic is closed, call an animal poison control hotline or go to the nearest emergency hospital.

Have this information ready when you call: your dog's weight, the date of the injection, the signs you are seeing, and when they started. That helps the veterinarian decide how quickly your dog needs to be seen.

Limitations and When to Contact a Veterinarian

This article covers general patterns. Your dog is an individual, and only a veterinarian who examines your dog can judge what a specific sign means. Call your veterinarian if you are unsure about anything you see after the injection.

Call the same day if your dog vomits more than once or twice, has diarrhea that will not stop, seems unusually tired, refuses food, or has swelling at the injection site that is getting bigger or feels hot.

Go to an emergency clinic right away if your dog collapses, has trouble breathing, has a seizure, develops hives or a swollen face, or cannot stand or walk normally.

Schedule a routine visit if a mild sign lingers longer than a day or two, or if you simply want your dog checked. It is always reasonable to ask for a recheck after a reaction, even a mild one.

Frequently Asked Questions

How long do ProHeart side effects last in dogs?

Most mild signs, such as vomiting or a sore injection site, settle within a day or two. Because the drug is released slowly, some dogs may feel off for a bit longer. If signs are still going strong after two days, call your veterinarian.

Can my dog still get heartworm while on ProHeart?

No preventive is 100 percent effective, but the injection is designed to kill heartworm larvae before they grow into adults. The risk goes up if a dose is late or missed. Keep your dog on schedule and test as your veterinarian recommends.

Should I switch to a monthly preventive if my dog reacted to the injection?

That depends on how serious the reaction was and what your veterinarian thinks caused it. A mild reaction may not mean your dog cannot use the product again. A serious reaction usually means your vet will choose a different preventive.

Are reactions more likely after the first injection or a later one?

Reactions can happen with any dose, including the first one. Some dogs do fine for years and then have a reaction later. That is why it helps to watch your dog after every injection, not only the first.

Can I give my dog anything at home before the injection to prevent a reaction?

Do not give any medication before the injection unless your veterinarian tells you to. Some drugs can interact with moxidectin or mask signs your vet needs to see. Ask your clinic what, if anything, is safe for your dog.

References

Milbemycin oxime and lotilaner for heartworm prevention in dogs in Korea: a randomized controlled field trial. Journal of veterinary science, 2026.
Comparing compressed 2-dose and 3-dose melarsomine protocols in the era of adjunctive therapy: a randomized clinical trial of 168 dogs with heartworm disease. Journal of the American Veterinary Medical Association, 2026.
A retrospective comparison of two-dose and three-dose melarsomine, with doxycycline and a macrocyclic lactone, in 1246 shelter dogs. Parasites & vectors, 2026.