Thiabendazole is an antiparasitic active ingredient used in veterinary medicine in livestock and pets against internal parasites (roundworms). It is also used against agricultural pests. It belongs to the chemical class of the benzimidazoles.


Other names: TBZ
Type: veterinary medecine
Chemical class: benzimidazole


Molecular structure of THIABENDAZOLE 



Type of action: broad-spectrum nematicidal anthelmintic, endoparasiticide
Main veterinary parasites controlled: gastrointestinal roundworms (= nematodes)

Efficacy against a specific parasite depends on the delivery form and on the dose administered. National regulatory authorities determine whether a product is approved for a given indication, i.e. use on a particular host at a specific dose and against a specific parasite. Check the labels of the products available in your country.

Click here for general information on features and characteristics of PARASITICIDES.


Oral LD50, rat, acute*: 3100-3600 mg/kg
Dermal LD50, rabbits, acute*: <5000 mg/kg
* These values refer to the active ingredient. Toxicity has to be determined for each formulation as well. Formulations are usually significantly less toxic than the active ingredients.

MRL (maximum residue limit) established for either beef, mutton pork or chicken meat*:

  • CODEX: Yes
  • EU: No
  • USA: Yes
  • AUS: Yes

* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock.

Withholding periods for meat, milk, eggs, etc. depend on delivery form, dose and national regulations. Check the product label in your country.

General information on the safety of veterinary antiparasitics is available in specific articles in this site (click to visit):


Never use products for livestock on dogs and cats unless they are explicitly approved for both livestock and pets. Pets may not tolerate livestock formulations

It is obvious that veterinary products are not intended for and should never be used on humans!!!


Decade of introduction: 1960
Introduced by: MERCK, SHARP & DOHME (=MS&D) → MERIAL
Some original brands: THIBENZOLE
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Yes, but very scarce
Use in HORSES: No, mostly abandoned
Use in DOGS and CATS: Yes, but very scarce

Main delivery forms: 

Use in human medicine: No
Use in public/domestic hygiene: No
Use in agriculture: Yes
Generics available:  Yes, rather few


In livestock: Yes, as all benzimidazoles, very frequent worldwide in gastrointestinal roundworms in sheep, goats and cattle.
In dogs and cats: No

Visit also the section in this site about parasite resistance to antiparasitics and more specifically to benzimidazoles.


Thiabendazole is a veteran anthelmintic (wormer) compound belonging to the chemical class of the benzimidazoles. It is one of the first generation benzimidazoles. Nowadays it is very scarcely used in livestock or pets: it has been vastly replaced by more modern compounds with a larger spectrum of efficacy.

In livestock thiabendazole was used in ruminants in the form of feed additives, drenches, bolus, tablets, pills, etc, all for oral administration, mostly alone. There are no classic injectables or pour-ons with thiabendazole.

Efficacy of thiabendazole

Thiabendazole has a broad-spectrum of activity against gastrointestinal roundworms, including adults and larvae of the most important species (e.g. of the genus Bunostomum, Haemonchus, Ostertagia - Teladorsagia, Trichostrongylus, Cooperia, Nematodirus, Chabertia, Oesophagostomum, etc.). Efficacy against migrating and arrested larvae is usually insufficient.

It is also effective against the major parasitic gastrointestinal roundworms (e.g. Ancylostoma, Toxocara, Trichuris, Uncinaria) of dogs and cats.

Thiabendazole has no residual effect. This means that a single administration will kill the parasites present in the host at the time of treatment, but it will not protect against re-infestations. This may require re-treatments to ensure proper control of certain parasites.

At the therapeutic dose thiabendazole is not effective against non-gastrointestinal roundworms (e.g. lungworms), tapewormsflukes and whatsoever external parasites.

Resistance of several gastrointestinal roundworms to all benzimidazoles, including thiabendazole is already very high and very frequent worldwide in sheep and goats, slightly lower in cattle. For this reason, the risk that benzimidazoles fail to protect ruminants against gastrointestinal roundworms is considerable worldwide. Resistance of worms to benzimidazoles in dogs, cats, pig and poultry is not a problem so far.

Thiabendazole is also effective against certain mycotic infections of livestock and pets.

Pharmacokinetics of thiabendazole

Unlike most benzimidazoles thiabendazole is rather soluble in water and following oral administration it is readily absorbed in the stomach. Up to 88% of the administered dose is absorbed to the bloodstream.

Absorbed thiabendazole is quickly metabolized in the liver to inactive metabolites, which are partly released back to the stomach. As a consequence it shows no activity against worms dwelling in the tissues or in other organs than the gastrointestinal tract. Another consequence is that concentration in the large intestine is usually insufficient to control worms there, e.g. Trichuris spp.

Blood levels of thiabendazole are higher in sheep than in goats and cattle.

Excretion is achieved mainly through urine (~90%) in the form of various metabolites, <5% through feces. Three days after treatment thiabendazole is almost completely excreted.

Mechanism of action of oxibendazole

The molecular mode of action of all benzimidazoles, including thiabendazole, consists in binding to tubulin, a structural protein of microtubules. These microtubules are important organelles involved in the motility, the division and the secretion processes of cells in all living organisms. In the worms the blocking of microtubules perturbs the uptake of glucose, which eventually empties the glycogen reserves. This blocks the whole energy management mechanism of the worms that are paralyzed and die or are expelled.

Since cell division is also disturbed, worm egg production and development is also blocked by benzimidazoles, i.e. most of them also have an ovicidal effect.

Click here to view the list of all technical summaries of antiparasitic active ingredients in this site.