Side Effects of Dewormer on Horses


All horses are exposed to parasites on a routine basis and should be dewormed several times a year. How often you deworm your horse depends on his risk of exposure to internal parasites. The most common internal parasites are strongyles (large and small) followed by ascarids (roundworms), bots, pinworms and tapeworms. The damage these parasites can do to your horse far outweighs the chance of side effects caused by dewormers.

What deworming does

When you deworm a horse you administer a chemical that kills the internal parasites in your horse. There are five chemicals routinely used in deworming: benzimidazoles, organophosphates, carbamates, piperazines, and ivermectin.

Each has a different efficacy and may only kill one type of parasite while another may target two or more. Many horse owners rotate the type of chemical they use, for example alternating a carbamate dewormer with an ivermectin dewormer.

Dewormers do not provide complete protection against internal parasites. Your horse may become reinfected or a type of parasite, primarily strongyles and ascarids, may become resistant to a specific dewormer.


Colic can occur when a horse with a high load of parasites is dewormed. If the majority of parasites die at the same time they can cause a blockage in the large intestine. If the blockage is left untreated the bowel may rupture causing almost instant death.

To avoid a massive die-off, have a fecal count (a sample of manure is floated in a special liquid, a slide is placed over the liquid and left for several minutes, it is then removed and parasite eggs that adhere to the slide are counted under a microscope) done before deworming. This will give you an idea of the parasite load your horse carries. For heavily infested horses using 1/2 of the recommended dose, waiting a week and giving the other half may help prevent an intestinal blockage and colic.

Always contact your veterinarian for advice if you are concerned about deworming your horse.


Some horses will develop allergies to certain deworming chemicals. Allergic horses will develop hives as quickly as fifteen minutes after deworming. These horses will rub against walls, trees and other solid structures in an attempt to relieve the itching. Their muzzles may swell and in extreme cases they may develop a fever and difficulty breathing.

If this happens contact your veterinarian immediately. He will administer anti-histamines and other medications to relieve the allergy symptoms.

Consult your veterinarian for advice on how to deworm a horse allergic to a dewormer.

Drug resistence

The biggest problem faced by horses and their owners is drug resistant internal parasites. There are parts of the United States where populations of strongyles and ascarids have become resistant to carbamate dewormers like pryrantel pamoate and are showing resistance to ivermectin.

To keep your horse healthy alternate the dewormer chemicals you use. By presenting parasites with different chemicals they will find it harder to become resistant to one particular dewormer.

How to prevent problems

You can prevent these problems by keeping your horse on a regular deworming schedule. Always monitor your horse after deworming for signs of colic or allergies. Have a fecal test done at least once a year to verify you are in fact reducing the number of internal parasites your horse has.

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