Side Effects of Heartworm Medicine


Heartworm, technically called dirofilaria immitis, is a parasite that is spread by mosquitoes and mostly affects dogs. Because these parasites live in the heart, an infection can be serious and sometimes fatal. Veterinarians usually recommend giving dogs a pill each month to prevent heartworms, particularly during months when the dogs are more likely to be bitten by mosquitoes.


Heartworm-prevention pills are actually small doses of curative substances that kill the early stage of heartworms. The pills contain an insecticide, which can be toxic over time.

Most Common Side Effects

Possible side effects of taking heartworm-prevention pills include vomiting, diarrhea, weakness and seizures.


Because the drugs are essentially small amounts of poison, the dog's liver and kidneys must work to eliminate the toxins, and this can weaken the immune system. Some veterinarians--like Will Falconer, writing for the K911 website--believe that long-term use of these drugs can result in arthritis, liver and kidney disease, and other degenerative disorders.

Injectable Heartworm Medicine

Injectable heartworm medicine, called Proheart, was recalled in 2004 after over 500 dog deaths, but was rereleased in 2008 with some modifications.

Injectable Medicine Side Effects

The injectable heartworm medicine has been shown to cause many possible side effects, including severe allergic reactions, blood abnormalities, seizures, diarrhea and vomiting.

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