What are the side effects of worm medicine for dogs?

Updated November 21, 2016

There are five general types of worms that dogs can contract: round worms, tapeworms, whipworms, heartworms, and hookworms. Some of these parasites are more devastating than others, but over time all of them can become a source of poor health and discomfort for your companion animal. It is important to recognise the side effects of the various worm medications. Once you do, you can make an educated decision about whether to treat a side effect as an emergency or as a worrisome but acceptable event.

General Side-Effects

Dogs with intestinal worms may appear lethargic, have distended stomachs, vomit, lose weight, experience diarrhoea, or become dehydrated. Once you have treated your animal, eventually these symptoms will abate. In the interim, however, they may as a side effect vomit (with or without visible worms), eliminate worms in their bowel movement, drool a little, or have more diarrhoea in moderate amounts. These effects shouldn't be severe and should abate in 24 hours. If they do not or you have questions about their severity, contact your veterinarian for professional advice.


De-worming medication is prescribed based on your dog's age and weight. If your dog receives an overdose she may experience lethargy, severe vomiting, excessive drooling, shaking or even seizures. If you notice any of these symptoms, immediately contact your veterinarian.


Heartworms, just as the name implies, are worms that infect the heart. These worms are contracted from carrier mosquitoes. If you live in a warm climate your dog is more likely to become infected than other dogs that live in cool climates. However, if your area has mosquitoes, then your dog runs the risk of contracting heartworms.

Veterinarians often prescribe monthly preventive medications such as ProHeart Rx, Heartgard Rx, and Interceptor Rx without known side-effects. Filaribits Plus is another prescribed monthly preventive. However, Filaribits has been implicated in liver problems in certain dogs, most notably Dobermans.

Treatment of active heartworm infestations has its own potential side-effects. If the case is mild, then using medications to kill the parasite is relatively safe. However, if the infestation is severe, your dog could die during treatment. This happens when the treatment medication kills too many of the worms at one time and the blood vessels that feed the heart become blocked by the dead worms. It is important for a heartworm infestation to be treated under close veterinary supervision.

Other possible side-effects include excessive coughing, your dog acting "down," fatigue, localised swelling and pain, drooling, diarrhoea, loss of appetite and bloody stool.

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