Heartworm Medication Side Effects

By Barb Nefer
Heartworm is a disease that can be contracted by dogs who are bitten by a carrier mosquito. Some mosquitos carry heartworm larvae and transfer it into a dog's bloodstream. The larvae mature into adults that live in the dog's heart, lungs and surrounding blood vessels, causing serious health problems. Martine Hartogensis, D.V.M, of the Food and Drug Association (FDA), says the purpose of heartworm medication is to protect a dog against being infected.


According to the FDA, there are three main types of heartworm medication for dogs, all of which are only available by prescription. There are oral forms that come in pill or tablet form, topical liquids that are placed on the dog's back and an injectable preventative.


Before using heartworm medication, a dog owner must have the pet tested by a veterinarian to make sure he is not already infected. If a dog that has heartworms is treated with a preventative, it can cause serious health problems. If the dog tests negative, the veterinarian will prescribe one of the available treatments. At the very least, the dog will be treated during mosquito season, which is longer in warm climates. However, many veterinarians recommend treatment all year.


Although heartworm disease was once confined to the southern United States, Doctors Foster and Smith reports that it is now found in all of the 48 continental states.

Oral Medication Side Effects

The most common side effects from heartworm medication in pill or tablet form include diarrhea, vomiting, decreased appetite and lethargic behavior. The effects are usually mild and often disappear over time.

Topical Side Effects

The most common side effects from topical liquid heartworm medication include skin irritation or dermatitis. These problems stop once the dog owner stops applying the liquid.

Injectable Side Effects

Moxidectin, an injectable heartworm medication, was temporarily withdrawn from the market because it caused death in some dogs. However, according to the FDA, it has been put back on the market with several restrictions. Veterinarians who inject it must have special training, explain the extra risks of this medication and have the client sign an informed content form.
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