The label says, word for word:
For use in dogs to prevent canine heartworm disease by eliminating the tissue stage of heartworm larvae (Dirofilaria immitis) for a month (30 days) after infection and for the treatment and control of roundworms (Toxocara canis, Toxascaris leonina) and hookworms (Ancylostoma caninum, Uncinaria stenocephala, Ancylostoma braziliense).
All dogs should be tested for existing heartworm infection before starting treatment with HEARTGARD PLUS which is not effective against adult D. immitis.
Infected dogs must be treated to remove adult heartworms and microfilariae before initiating a program with HEARTGARD PLUS.
Choking or intestinal obstruction has been reported after dosing with HEARTGARD PLUS.
HEARTGARD PLUS is recommended for dogs 6 weeks of age and older.
Keep this and all drugs out of the reach of children. In case of ingestion by humans, clients should be advised to contact a physician immediately.
Every approved medicine has a field study behind it, and the label prints what happened:
In clinical field trials with HEARTGARD PLUS, vomiting or diarrhea within 24 hours of dosing was observed (1.1% of administered doses).
Post-Approval Experience: The following adverse events reported in dogs are listed in decreasing order of reporting frequency: Vomiting, diarrhea, lethargy, anorexia, seizures, ataxia, muscle tremors, hypersalivation, pruritus.
In some cases, choking or intestinal obstruction has been reported after administration of HEARTGARD PLUS.
Approved by FDA under NADA 140-971. The FOI summary for NADA 140-971 (https://animaldrugsatfda.fda.gov/adafda/app/search/public/document/downloadFoi/542, product then named HEARTGARD-30 Plus, sponsor Merck Research Laboratories) contains no approval date, so approved_year is null. The label has no Contraindications section and no Client Information Sheet; the heartworm-testing and choking statements appear under Precautions.
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. It is a chewable given to dogs once a month to prevent heartworm disease and to treat and control roundworms and hookworms.
The label gives no time. It says the chewable is effective against the young stage of the heartworm for a month (30 days) after the dog was infected, and that this is how it prevents adult heartworms from developing. Ask the vet what to expect and by when.
It is given by mouth once a month. The label says the chewable can be offered by hand or added whole to a small amount of dog food. The dog should chew it instead of swallowing it whole, because choking and a blocked intestine have been reported. For a dog that normally swallows treats whole, the label says the chewable may be broken into pieces. Watch your dog for a few minutes afterwards, and if you think part of the dose was lost, the label recommends giving the dose again. It is given every month during the part of the year when mosquitoes are active. The first dose is due within a month (30 days) of the dog's first exposure to mosquitoes, and the last dose within a month (30 days) of the last exposure. The label says all dogs should be tested for heartworm infection before they start.
The label says the protection can be reduced when more than a month (30 days) passes between doses, so the chewable should be given on or about the same day every month. If a dose is late, "whether by a few days or many", the label says that giving the chewable at once and going back to the monthly schedule keeps the chance of adult heartworms developing as small as possible.
The studies and the warnings are quoted above. The label has no line telling owners when to call the vet. It reports vomiting or diarrhea within 24 hours of a dose in its field trials, and choking or a blocked intestine after dosing. If your dog seems unwell after a dose, call the vet. If a person swallows a chewable, the label says to contact a physician immediately.
Store it at room temperature, between 68° and 77°F. The label allows short spells between 59° and 86°F. Protect it from light: take out one chewable at a time and put the card with the rest back in its box. Keep it out of the reach of children.
The FDA says the best way to get rid of most unused or expired medicines 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, the FDA says a medicine that is not on its flush list can go in the trash at home. Its steps are to mix the medicine with something unappealing such as dirt, cat litter or used coffee grounds, without crushing it, 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: _____________________________________________