Horse Side Vet Guide ®

Equine Health Resource

Deworming, Benzimadazole



Benzimidazoles have been a mainstay of equine parasite control for over 40 years. These chemicals interfere with a worm’s energy metabolism on a cellular level, causing a slower kill of the parasites than the so-called “paralytic compounds”.

Familiar examples of benzimidazoles are fenbendazole (Panacur®) and oxibendazole (Anthelcide® EQ). These continue to have good activity against Ascarids and large strongyles, but small strongyles are now mostly resistant to this class.

For this reason, these drugs should probably only be used in young horses. They should not be used as part of a rotation unless recommended by your veterinarian based on fecal results. Further random use will only further contribute to parasite resistance against this class.

Benzimidazoles are relatively safe in all ages of horses and unlikely to cause side effects even at higher doses.

These drugs require prolonged contact time with internal parasites in order to kill them effectively. This is why repeated doses may increase their effectiveness. A commercial, high dose, daily 5-day product has been marketed as being effective against encysted strongyles, but there is disagreement about its efficacy.

There are a number of different compounds of this drug class sold commercially. The most common of these are oxibendazole and fenbendazole. These drugs are most commonly sold as paste formulations. Liquid formulations are sometimes used at high dosages given by stomach tube by veterinarians.

Benzimidazoles do still have some application, especially against ascarid parasites in young horses.


The usefulness of any anthelmintic medication depends not only on the effectiveness of the drug against a particular parasite species, but also on the drug’s pharmacology (how much of the chemical is exposed to the parasite in question and for how long), characteristics of the host animal (general health and immunity) and characteristics of the parasite (not only its susceptibility to the anthelmintic but also its location in the body and susceptibility at various life stages to the drug).

Your vet’s role is to determine if and when benzimadazole products should be used and to determine their effectiveness following use with fecal testing (fecal egg count reduction test).


Horse owners play a vital role in the use of anthelmintic drugs. All anthelmintic drugs should be administered as part of a targeted deworming program along with management techniques to reduce intake of infective parasites.

Random rotational use of dewormers without fecal testing is leading to parasite resistance and the rapid loss of effectiveness of this and other deworming compounds.

Is It working? Timeframe for effect

In most cases, it is difficult to tell whether or not a deworming compound is effective. In many cases, you will not see dead worms in the manure after deworming.

Just because a horse appears well does not mean that it does not have a large parasite load.

The only reliable way to determine the effectiveness of a particular deworming compound is to perform a fecal egg count reduction test (FECRT). A fecal egg count is performed on the horse. The dewormer is administered. Two weeks later, a second fecal egg count is performed. If the dewormer is effective, then the egg count of the second sample should be zero. Talk to your vet about creating a proper deworming protocol for your horses.

Questions To Ask My Vet

  • Do benzimadazole products have any place in my parasite control strategy?
  • How often should you do fecal exams?
  • What management changes should I make to reduce the worm burden in my horses?
  • When should fecal testing be performed to determine the effectiveness of my deworming program?
Merck Animal Health Safe-Guard® Dewormer 20% Type A Medicated, Deworming
Merck Animal Health Panacur® Powerpac (10% fenbendazole), Deworming
Merck Animal Health Panacur® Paste (10% fenbendazole), Deworming
Merck Animal Health Safe-Guard® Equine Dewormer (10% fenbendazole), Deworming
Merck Animal Health Safe-Guard® Power Dose®, Deworming
Zoetis Animal Health Anthelcide® EQ, Deworming
Zoetis Animal Health Anthelcide® EQ Suspension (oxibendazole), Deworming
Zoetis Animal Health Anthelcide® EQ Paste (oxibendazole), Deworming
Merck Animal Health Panacur® Horse Dewormer Suspension 10%, Deworming
Merck Animal Health Panacur® Power Pac Equine Dewormer, Deworming
Merck Animal Health Safe-Guard® Equi-bits® Top Dress Pellets, Deworming
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Author: Doug Thal DVM Dipl. ABVP


Bowman DD. Georgis' Parasitology for Veterinarians 9th Ed. Elsevier 2009.


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