ProHeart for Dogs: Age Requirements and When It Can Start
By Dr. Zubair Khalid, DVM, MS, PhD ·
Puppies can usually receive their first ProHeart injection at 8 weeks of age, and many veterinarians start then if the puppy is healthy and heartworm negative. Adult dogs can start at any age, but most need a negative heartworm test first. The exact timing for your dog depends on age, test results, and your veterinarian's judgment.
This article is educational and is not a substitute for veterinary diagnosis or treatment.
Quick Answer
- ProHeart injectable heartworm prevention is generally approved for puppies starting at 8 weeks of age, though your veterinarian confirms your puppy is healthy enough.
- Puppies younger than 6 months typically do not need a heartworm test before starting because any infection from a mosquito bite would be too young to show up on tests.
- Dogs 6 months and older usually need a negative heartworm antigen test before the first injection.
- ProHeart 6 is given every 6 months and ProHeart 12 is given once a year, so the injection schedule affects when your dog needs the next dose.
- If your dog has missed monthly heartworm pills or has an unknown prevention history, your veterinarian will test before giving ProHeart.
What ProHeart Is and What Veterinarians Use It For
ProHeart is an injectable heartworm preventive for dogs. It contains moxidectin, a macrocyclic lactone medication that kills heartworm larvae before they can grow into adult worms in the heart and lungs. Instead of a monthly chewable or topical product, your dog receives a single injection in the veterinary clinic that protects against heartworm infection for a set period.
Veterinarians use ProHeart for two main reasons. First, it removes the daily or monthly pill burden that leads many owners to miss doses. Second, it gives reliable, long-acting protection for dogs whose owners travel, work long hours, or simply forget monthly products. Owner noncompliance, not preventive failure, drives most canine heartworm infections in established patients, and only about 24.4% of dogs under veterinary care achieve full-year purchase compliance with monthly macrocyclic lactones [1]. An injectable option sidesteps that problem.
ProHeart does not treat adult heartworm disease. It prevents infection by killing the immature larval stages. If your dog already has adult heartworms, a different treatment plan is needed, and your veterinarian will discuss that separately.
How ProHeart Works
When a mosquito bites an infected dog, it picks up heartworm microfilariae and later deposits infective larvae into the next dog it bites. Those larvae migrate through the body and mature into adult worms over several months. Moxidectin kills the larvae during this migration window, before they reach the heart and pulmonary arteries.
Because the injection releases medication slowly over months, your dog has continuous protection between visits. That continuous chemical protection is the goal, not annual detection of past failures [1]. This is why veterinarians emphasize keeping your dog on prevention every month of the year, even in cooler climates where mosquito activity drops.
ProHeart does not stop mosquitoes from biting your dog. It stops the larvae from surviving after a bite. Your dog still needs other parasite prevention for fleas, ticks, and intestinal worms as your veterinarian recommends.
ProHeart for Dogs Age: Minimum Age and Puppy Eligibility
This is the question most owners ask first. The minimum age for ProHeart in puppies is 8 weeks. Puppies younger than that should not receive the injection. Your veterinarian will confirm your puppy is healthy, eating well, and gaining weight before giving the first dose.
ProHeart minimum age puppies
At 8 weeks, a puppy's body can handle the medication, and the injection begins protecting against heartworm larvae right away. Many veterinarians pair the first ProHeart injection with a puppy wellness visit that also includes vaccines and a fecal check. If you are planning that first visit, our New Puppy Checklist: The First 30 Days walks through what to expect.
Timing matters because puppies are at risk as soon as they are outside and exposed to mosquitoes. Starting prevention early closes that gap.
ProHeart puppy eligibility and testing
Puppies under 6 months of age generally do not need a heartworm antigen test before starting ProHeart. The reason is simple. Antigen tests detect adult female heartworms, and it takes about 6 months after infection for adults to develop and be detectable. A puppy infected last week will test negative even though larvae are present. So testing a young puppy adds cost without useful information.
Once a dog reaches 6 months of age, the picture changes. Current American Heartworm Society guidelines characterize prescribing macrocyclic lactone preventives in the absence of antigen testing as "NOT RECOMMENDED," regardless of compliance history or geographic prevalence [2]. In plain terms, your veterinarian should test your dog before starting or renewing prevention once your dog is old enough for a test to be meaningful.
What about adult dogs starting ProHeart for the first time?
Adult dogs of any age can start ProHeart if they test negative for heartworm. Your veterinarian will run an antigen test, and sometimes a microfilaria test, before the first injection. If your dog has been on monthly prevention without gaps, some veterinarians still require the test at renewal, and the guidelines support that practice [2]. If your dog has missed doses or has an unknown history, testing is essential.
Puppy protocols and the first year
A typical puppy protocol looks like this. The first ProHeart injection happens at 8 weeks or later. If your puppy receives ProHeart 6, the next injection is due 6 months later. If your puppy receives ProHeart 12, the next injection is due in 12 months. Around the 6 to 7 month mark, your veterinarian will add a heartworm antigen test to confirm your puppy is negative before continuing prevention. This test catches any infection that occurred before prevention started.
Some veterinarians prefer monthly preventives for very young puppies and switch to injectables once the puppy is older. Both approaches are reasonable. Your veterinarian will pick based on your puppy's health, your schedule, and local heartworm risk.
Dosing Basics: Who Decides, How It Is Given, and Missed Doses
Your veterinarian calculates the ProHeart dose from your dog's body weight and health status. The dose is not something you measure at home. ProHeart is given as an injection in the clinic, usually under the skin or into the muscle, depending on the product. Your dog does not need sedation for the injection.
Because the dose depends on weight, your veterinarian will weigh your dog at each visit. Puppies grow fast, so the dose changes as they grow. If your dog gains or loses a significant amount of weight between visits, tell your veterinarian.
What if you miss a ProHeart dose?
ProHeart injections are given on a schedule, either every 6 months or every 12 months. If you miss the due date, call your veterinarian right away. The protection window does not extend past the labeled interval. A late injection means your dog had a gap in coverage, and your veterinarian may recommend a heartworm test before restarting. Do not wait until the next annual exam if you know the injection is overdue.
What if your dog was on monthly pills and you want to switch?
Switching from monthly prevention to ProHeart is common. Your veterinarian will review your dog's prevention history, test for heartworm if your dog is 6 months or older, and then give the injection. In most cases, the last monthly dose and the first injection are timed so there is no gap in protection. Your veterinarian will tell you the exact timing for your dog.
Side Effects: Common and Serious
Most dogs tolerate ProHeart well. Some experience mild reactions at the injection site or mild sluggishness for a day. Serious reactions are uncommon but possible. The table below summarizes what owners typically see.
| Side effect | How common or serious | What to do |
|---|---|---|
| Soreness, swelling, or a small lump at the injection site | Common and usually mild | Watch for a few days. Call your veterinarian if swelling grows, is painful, or does not improve. |
| Sleepiness, decreased appetite, or mild vomiting in the first day or two | Uncommon and usually mild | Offer water and rest. Call your veterinarian if signs last more than 48 hours or worsen. |
| Hives, facial swelling, or itching | Uncommon but serious | Call your veterinarian right away. This may need treatment. |
| Difficulty breathing, collapse, or severe weakness | Rare but life threatening | This is an emergency. Go to the nearest veterinary hospital immediately. |
| Persistent vomiting, diarrhea, or lethargy | Uncommon | Contact your veterinarian the same day for advice. |
If your dog has a history of reactions to moxidectin or other macrocyclic lactones, tell your veterinarian before the injection. Your veterinarian may choose a different preventive.
Who Should Not Take ProHeart and Drug Interactions
ProHeart is not right for every dog. Your veterinarian will review your dog's history before prescribing it.
Dogs who should not receive ProHeart include those with a known allergy to moxidectin or related medications, dogs who are currently heartworm positive (the injection does not treat adult infections), and dogs who are sick, weak, or underweight at the time of the visit. Puppies younger than 8 weeks should not receive it.
Tell your veterinarian about every medication, supplement, and preventive your dog takes. Some drugs can interact with moxidectin, including certain products used for parasites. Your veterinarian will check for interactions before giving the injection. If your dog is on a monthly preventive and you want to switch, do not give both at the same time unless your veterinarian directs it.
Dogs with heart disease need extra care. If your dog has a heart murmur, exercise intolerance, or a known cardiac condition, your veterinarian may want to evaluate the heart before starting prevention. Our article on Dog Heart Disease: Recognizing the Early Signs and Knowing When to Act explains what those signs look like at home.
Overdose and When to Call a Veterinarian or Poison Control
ProHeart is given by injection in a clinic, so overdose at home is not a concern with this product. Accidental overdose would happen only if a dog received too much medication during a veterinary visit, which is rare because the dose is calculated by weight.
If you think your dog received too much moxidectin, or if your dog has a severe reaction after an injection, call your veterinarian immediately. For after-hours emergencies, call the ASPCA Animal Poison Control Center at (888) 426-4435 or the Pet Poison Helpline at (855) 764-7661. Both services may charge a consultation fee. Have your dog's weight, the product name, and the time of the injection ready when you call.
Signs that need urgent care include collapse, seizures, severe vomiting, difficulty breathing, or a swollen face. Do not wait to see if these signs pass on their own.
Limitations and When to Contact a Veterinarian
ProHeart prevents heartworm infection, but it does not treat adult heartworms, and it does not protect against every parasite. It also depends on you keeping the injection schedule. A late injection leaves a gap.
Call your veterinarian the same day if your dog has vomiting, diarrhea, or lethargy that lasts more than a day after the injection, or if the injection site is swollen and painful.
Go to an emergency clinic now if your dog has trouble breathing, collapses, has a seizure, or develops sudden facial swelling or hives.
Schedule a routine visit if your dog is due for a ProHeart injection, if you missed a dose, or if you are switching from monthly prevention. Your veterinarian will test and set the next date.
Every dog is different. Your veterinarian needs to examine your dog and review the history before deciding whether ProHeart is right and when to start.
Frequently Asked Questions
Can a puppy get ProHeart before 8 weeks of age?
No. The minimum age is 8 weeks, and your veterinarian will confirm your puppy is healthy before giving the injection. Starting earlier is not recommended because the medication has not been evaluated in younger puppies.
Does my puppy need a heartworm test before the first ProHeart injection?
Puppies under 6 months old generally do not need a test because infection would be too recent to detect. Once your dog reaches 6 months, your veterinarian will test before continuing prevention.
Can my dog start ProHeart if she has been off heartworm prevention for a year?
Possibly, but she needs a heartworm test first. If the test is negative, your veterinarian can start the injection. If it is positive, your dog needs treatment for adult heartworms before prevention restarts.
Is ProHeart safe for senior dogs?
Many senior dogs can receive ProHeart if they are healthy and test negative for heartworm. Your veterinarian will weigh your dog's overall health, medications, and organ function before deciding.
What happens if I am late for a ProHeart injection?
Call your veterinarian as soon as you realize the injection is overdue. A gap in protection means your dog may need a heartworm test before the next injection, and your veterinarian will reset the schedule from that visit.
Related Articles
- Puppy Training Guide By Age
- New Puppy Checklist: The First 30 Days
- Dog Heart Disease: Recognizing the Early Signs and Knowing When to Act
- Pro BNP Test for Feline and Canine Heart Disease: Screening and Limits
- When to Spay or Neuter a Puppy: Optimal Age and Health Considerations
- When to Spay a Dog: Optimal Age for Small vs Large Breeds
- ProHeart Injection for Dogs: How It Works and Dosing
- ProHeart 6 vs ProHeart 12 for Dogs: What Owners Should Know
- ProHeart Injection Side Effects in Dogs: What to Watch For
- Canine Heartworm and Flea Prevention: Combined Oral Medications and Clinical Guidelines
- Canine Heartworm Prevention and Flea Control: Combination Pill Options and Veterinary Guide
References
- Are common clinical protocols prioritizing detection over prevention in canine Dirofilaria immitis?. Journal of the American Veterinary Medical Association, 2026.
- Is annual Dirofilaria immitis antigen testing at preventive renewal proportionate to the residual infection risk in documented-compliant dogs?. Journal of the American Veterinary Medical Association, 2026.