The label says, word for word:
ProHeart 6 is indicated for use in dogs six months of age and older for the prevention of heartworm disease caused by Dirofilaria immitis for 6 months. ProHeart 6 is indicated for the treatment of existing larval and adult hookworm (Ancylostoma caninum and Uncinaria stenocephala) infections.
ProHeart 6 is contraindicated in animals previously found to be hypersensitive to this drug or ProHeart 12.
ProHeart 12 is contraindicated in animals previously found to be hypersensitive to this drug or ProHeart 6.
Anaphylactic and anaphylactoid reactions may occur in some dogs following administration of ProHeart 6 alone or with vaccines. In some cases, these reactions have resulted in death following administration of moxidectin microspheres.
Do not administer ProHeart 6 to dogs who are sick, debilitated, underweight or who have a history of weight loss.
Caution should be used when administering ProHeart 6 in dogs with pre-existing allergic disease, including food allergy, atopy, and flea allergy dermatitis.
Always provide Client Information Sheet and review with owners before administering ProHeart 6. The owner should be advised to observe their dog for adverse drug events including those described on the sheet.
Only veterinarians and veterinary technicians/assistants that have completed the training and are certified can administer ProHeart 6 and ProHeart 12.
Every approved medicine has a field study behind it, and the label prints what happened:
In field studies, the following adverse reactions were observed in dogs treated with ProHeart 6: anaphylaxis, vomiting, diarrhea (with and without blood), listlessness, weight loss, seizures, injection site pruritus, and elevated body temperature.
Dogs with clinically significant weight loss (>10%) were more likely to experience a severe adverse reaction.
ProHeart 12 field study table (ProHeart 12 group, 297 dogs): Vomiting 75 (25.3%); Lethargy 46 (15.5%); Diarrhea (with and without blood) 43 (14.5%); Anorexia 41 (13.8%); Seizures 10 (3.4%); Hepatopathy 8 (2.7%); Hypersalivation 7 (2.4%); Anaphylactoid/Hypersensitivity Reactions 6 (2.0%)
From the sheet the maker writes for owners:
ProHeart 6: 'Approved by FDA under NADA # 141-189'; FDA states 'ProHeart 6 was approved in 2001'. ProHeart 12: 'Approved by FDA under NADA # 141-519', approval announced by FDA July 2, 2019; DailyMed setid e856da36-f717-4645-8841-27460cf87d91; indication (verbatim): 'ProHeart 12 is indicated for use in dogs 12 months of age and older for the prevention of heartworm disease caused by Dirofilaria immitis for 12 months.' and 'ProHeart 12 is indicated for the treatment of existing larval and adult hookworm (Ancylostoma caninum and Uncinaria stenocephala) infections.'
The emergency page sorts the signs by how long you have, and the symptom checker asks what you are seeing. Reactions to a medicine can also be reported to the FDA yourself: how to report a problem.
These answers come from the approved label on DailyMed, read on 2 October 2026. Where the label does not say, the answer says so, and the vet or the pharmacist is the one to ask.
The approved use is quoted at the top of this page. The label read for these answers is the one for ProHeart 6, an injection for dogs six months of age and older that prevents heartworm disease for six months and treats hookworm infections the dog already has. ProHeart 12 has a label of its own.
The label says one injection prevents heartworm disease for six months, and that the level of the drug in the blood peaks about 7 to 14 days after the injection. For hookworms, it says the injection clears the worms present at the time of treatment, that lasting protection has not been established, and that a dog can be infected again sooner than six months.
You do not give it yourself. It is an injection under the skin given at the clinic, and the label says only vets and vet technicians or assistants who have completed the training and are certified can give it. The label says the dog should be tested for heartworm first, and it tells the vet to give you the Client Information Sheet and go through it with you before the injection. It says the injection should be given within one month of the dog's first exposure to mosquitoes or, when it replaces another heartworm preventive, within one month of the last dose of that product. Follow-up injections may be given every six months, and the label says not more often than that.
The label does not say what to do about a missed or late injection. Call the clinic and ask when the next one should be.
The studies and the warnings are quoted above. The label tells vets to advise owners to contact them immediately if they see signs of a reaction. The reactions it lists include a severe allergic reaction, vomiting, diarrhea with or without blood, listlessness, weight loss, seizures, itching where the injection went in and a raised body temperature. Its warning says severe allergic reactions have in some cases ended in death.
The storage rules on the label are for the clinic, which keeps and mixes the product. Before mixing it is stored at or below 77°F and kept away from long exposure to light; once mixed it is kept in a refrigerator at 36° to 46°F, where the label says it is stable for 8 weeks.
There should be no leftovers of this medicine at home, because the injection is given at the clinic. For the other medicines in the house, the FDA says the best way to get rid of most unused or expired ones is a take-back option: a drug take-back location, which can be at a pharmacy or a police station, or a prepaid mail-back envelope. If you cannot use one and the medicine is not on the FDA's flush list, the FDA says to mix it, uncrushed, with something unappealing such as dirt, cat litter or used coffee grounds and to seal the mixture in a plastic bag before throwing it away. Flushing is only for medicines on that list, such as opioids. Do not give leftovers to another pet.
Write the dose, the times and the stop date here, then print this page for the fridge.
Dose and times: _____________________________________________
Stop date: _____________________________________________