ProHeart is an injectable heartworm prevention for dogs that provides either 6 or 12 months of protection from a single shot. Unlike monthly chewables or topicals, it’s given by a veterinarian as a subcutaneous injection and uses a slow-release formulation that steadily delivers the active ingredient moxidectin over the full protection period. It’s 100% effective at preventing heartworm disease for its labeled duration and also treats existing hookworm infections at the time of the shot.
How ProHeart Works
The injection deposits tiny microspheres under the skin near the base of your dog’s neck. These microspheres gradually dissolve and release moxidectin into the bloodstream over months. Moxidectin kills heartworm larvae (the immature stage transmitted by mosquitoes) before they can mature into adult worms that damage the heart and lungs. The continuous release means there’s no gap in protection, which is the main advantage over monthly preventives where a missed or late dose can leave your dog vulnerable.
ProHeart also eliminates hookworms present at the time of injection, including both larval and adult stages. In lab studies, it was at least 90% effective against the two most common hookworm species in dogs. It does not, however, cover other intestinal parasites like roundworms, whipworms, or tapeworms, so your vet may recommend additional deworming depending on your dog’s risk.
ProHeart 6 vs. ProHeart 12
The two versions differ in duration and age eligibility. ProHeart 6 lasts six months and can be given to dogs 6 months of age or older. ProHeart 12 lasts a full year and is approved for dogs 12 months of age or older. ProHeart 12 contains three times the concentration of the same formulation. Both use the same active ingredient and slow-release technology.
For most adult dogs, ProHeart 12 is the more convenient option since it aligns with annual vet visits. ProHeart 6 is useful for younger dogs who aren’t yet old enough for the 12-month version, or in situations where a shorter commitment makes more sense.
Who Can Give the Injection
You can’t buy ProHeart or administer it at home. Federal law restricts it to licensed veterinarians, and beyond that, the product is only available through a restricted distribution program. Vets and their staff must complete a specific training and certification module before they’re allowed to administer either version. This extra layer of oversight exists because of the potential for serious allergic reactions, which require immediate veterinary response.
Effectiveness
In both laboratory and field studies submitted to the FDA, ProHeart 12 was 100% effective at preventing heartworm disease for 12 months. A field study followed 297 dogs who received the injection, and none developed heartworm. In a controlled lab study, dogs were deliberately exposed to heartworm larvae 365 days after treatment, and the injection prevented every single infection.
Common Side Effects
In the main clinical trial of 593 dogs, the side effects seen in ProHeart-treated dogs were similar in frequency to those in dogs receiving a standard monthly oral preventive. About 25% of dogs in both groups experienced vomiting at some point during the study. Roughly 15% showed lethargy, and about 14% had diarrhea. Loss of appetite occurred in about 14% of ProHeart dogs compared to 10.5% in the control group.
Mild injection site reactions (warmth, swelling, or itching) were reported in a small number of dogs and typically appeared within a week of the shot.
Serious Reactions to Watch For
The most significant risk with ProHeart is an allergic reaction, which can range from hives and facial swelling to full anaphylaxis. These reactions are uncommon but can be severe. In rare cases reported after the product went to market, allergic reactions have been fatal. This is the primary reason the product requires certified veterinary administration: if a reaction happens, the vet team can intervene immediately.
Other serious but uncommon reactions reported after widespread use include immune-related blood disorders, liver problems (elevated liver enzymes), seizures, and difficulty breathing. The post-market reporting also includes cases of weakness, trembling, and uncoordinated movement. These events are rare relative to the large number of dogs that receive the injection, but they’re part of the product’s documented safety profile.
Dogs that already have heartworm infections may react more strongly. In a lab study, dogs carrying 4- to 6-month-old heartworm infections experienced vomiting, lethargy, and bloody diarrhea after receiving the injection. Your vet will test your dog for heartworm before administering ProHeart.
Safety in Sensitive Breeds
Some herding breeds, particularly Collies, Australian Shepherds, and related dogs, carry a gene mutation (MDR1) that makes them sensitive to certain parasite-prevention drugs. Moxidectin belongs to the same drug class that can cause problems in these breeds at higher doses. However, the manufacturers tested ProHeart specifically in dogs with the MDR1 mutation, and the FDA has confirmed that the product is safe for these dogs when used at the labeled dose. If your dog is a breed known to carry this mutation, ProHeart at its standard dose is not a concern.
What to Expect at the Vet
Before your dog receives ProHeart, the vet will run a heartworm test to confirm your dog is negative. The injection itself is quick, delivered just under the skin near the shoulder blades. Most dogs tolerate it without much fuss. Your vet will likely ask you to stay for a short observation period afterward to watch for any immediate allergic response.
After the visit, keep an eye on your dog for the first 24 to 48 hours. Mild lethargy or a small bump at the injection site is normal. Signs that warrant a call to your vet include facial swelling, hives, difficulty breathing, persistent vomiting, or sudden collapse. With ProHeart 12, your next dose lines up with the following year’s annual exam, making it one of the lowest-maintenance heartworm prevention options available.