Why Does My Dog Get Sick From Heartworm Medicine?

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As a dog owner, you are likely aware of how important it is to have your dog on a heartworm preventative. Adult heartworms can live for up to seven years, and in that time, they can do a lot of damage to your dog's heart and lungs. Many preventative products also contain dewormer, such as pyrantel, to treat intestinal parasites, like hookworms and roundworms. One of them even treats ear mites. But as with any medication, some pets may not tolerate them well. Heartworm medication side effects are rare and can range from mild to serious.


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What is heartworm disease?

Heartworm disease is what occurs after a dog is infected with the blood parasite Dirofilaria immitis.‌ Heartworm has been reported in all 50 states and can be found around the world. The parasite D. immitis is transmitted into a dog's bloodstream when an infected mosquito bites and feeds on a dog. Heartworm does not spread directly from dog to dog.


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There are four stages (or classes) of dog heartworm disease, ranging from mild (no symptoms or an occasional cough) to severe, where a major blood vessel connected to the heart is partially blocked by adult worms.

An infected dog may have symptoms that include:


  • Trouble breathing
  • Coughing
  • Weakness
  • Exercise intolerance
  • Enlarged liver
  • Abdominal fluid
  • Weight loss
  • Heart failure
  • Death

How is heartworm disease treated?

With the exception of dogs with the most severe stage of heartworm disease (class 4), dogs with class 1, 2, or 3 are treated with a combination of injectable and oral medications‌. A series of injections of a drug called melarsomine is given to kill adult heartworms. Baby worms, called microfilaria, are killed with certain heartworm preventatives.


Dogs may receive other medications as well, such as prednisone and antihistamines, to reduce the body's reaction to dying worms. Heartworm treatment can last three months or longer. Dogs with class 4 disease (also called caval syndrome) need to have worms surgically removed. Caval syndrome is life-threatening even if worms are removed.


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Can heartworm medicine make a dog sick?

Like any medication, heartworm medicine can make some dogs sick.‌ However, the overall risk of side effects is low. When used appropriately, the FDA-approved canine heartworm preventatives on the market are considered to be some of the safest medications in veterinary medicine. They are also considered safe to use in dogs receiving a variety of vaccinations, antibiotics, flea preventatives, and dewormers.



Heartworm medication side effects in dogs

Various rare side effects have been reported for heartworm preventative medications. Some of them are:

  • Weakness
  • Loss of appetite
  • Vomiting
  • Diarrhea
  • Hypersalivation (drooling)
  • Staggering gait
  • Convulsions
  • Death (ProHeart)


Due to a genetic mutation called MDR1, dog breeds such as collies and Australian shepherds are more sensitive to ivermectin. Heartworm preventatives whose active ingredient is ivermectin are safe to use in these breeds when administered according to the package directions. Giving a collie breed more than the recommended dose can cause symptoms ranging from drooling and weakness to tremors, coma, and death.


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How does heartworm medication work?

The way heartworm preventative medication works is that it kills off any heartworm larvae (baby worms) circulating in your dog's body before they get to the adult stage, which is when they cause damage.‌ The amount of medication needed is small, and it's typically administered orally or topically once a month. There is also an extended-release injectable medication called ProHeart that is administered by a DVM every six to 12 months.

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How to administer heartworm preventative

There are various heartworm medications available, and it's best to consult your veterinarian on which product to use. Most people opt for a preventative they can give at home. These come in chewable tablets, pills, and topical liquids. Some veterinary clinics carry injectable ProHeart 6 and ProHeart 12. These injections must be administered by a veterinarian.


Puppies should be started on a heartworm preventative at 6 weeks of age. Since it takes six to nine months after infection for a dog to test positive for heartworm, puppies under 6 months of age can be started on a preventative without a blood test. But puppies started on a preventative after 8 weeks of age should have their first heartworm test after 6 months. Dogs 7 months of age and older need to have a negative heartworm test first. Annual testing for dogs is recommended because of the potential for a missed dose, a late dose, or a dose that was spit out or rubbed off. For the best protection, pet owners should give the preventative year-round.

If your dog is growing, make sure to monitor their weight. In order to be protected, your pup needs to be receiving the correct amount of medication for their weight.

The most common heartworm medications are:

  • HeartGard
  • HeartGard Plus
  • Interceptor Plus
  • Iverhart Plus
  • Iverhart Max
  • Revolution
  • Tri-Heart Plus
  • Sentinel
  • Trifexis
  • Advantage Multi

The bottom line

Heartworm prevention is extremely important in protecting your dog, especially if they spend a lot of time outside. However, even indoor dogs can get a heartworm infestation. Every year, along with your dog's annual checkup and vaccinations, be sure to have them tested for heartworm. This involves a simple blood test. Although there are some rare adverse reactions to heartworm medication, prevention is far safer and significantly less expensive than having to treat your dog for a heartworm infection.