Fenthion is an antiparasitic active ingredient used in veterinary medicine in dogs and livestock against external parasites (lice, mites, fleas, fliesticks, etc.). It is also used against agricultural pests. It belongs to the chemical class of the organophosphates.

Common name: FENTHION

Type: pesticide
Chemical class: organophosphate


Molecular structure of FENTHION






Type of action: Broad spectrum, contact, non-systemic ectoparasiticide: insecticide, acaricide, tickicide, louisicide, larvicide
Main veterinary parasites controlled: flies, lice, fleas, mosquitoes, ticks, mites, fly maggots (cutaneous myiasis), etc.

Efficacy against a specific parasite depends on the delivery form and on the dose administered. 

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Fenthion is a broad-spectrum organophosphate highly effective against many parasitic insects (flieslice, fleas, mosquitoes, cattle grubs, human bot flies, etc.) but is completely ineffective against internal parasites when administered topically (i.e. on the skin).

As most organophosphates, fenthion products are used exclusively for topical administration, in this case mostly in the form of a spot-on, both for livestock and pets. Nowadays usage has strongly declined because it has been replaced by newer, more effective and less toxic compounds.

It is also used against agricultural and household pests.

However, resistance to all organophosphates is already very frequent worldwide, particularly in cattle ticks, houseflies, mosquitoes, fleas, blowfly strike, etc.

The table below indicates some usual dosing recommendations for fenthion issued by manufacturers or documented in the scientific literature. They may not be approved in some countries.

Dosing recommendations for FENTHION
Delivery Parasites Dose (against fenthion-susceptible parasites)
Spot-on Fleas, lice
8-25 mg/kg dep. on animal's weight
Spot-on Fleas, lice 6-15 mg/kg dep. on animal's weight
Spot-on Flies, human bot flies 4.5-15 mg/kg dep. on animal's weight
Spot-on Licecattle grubs 4.5-8 mg/kg dep. on animal's weight
Spot-on Lice 10 mg/kg
Spray Lice 9 mg/kg

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Dosing recommendations for antiparasitics depend on national regulations. 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 parasiteCheck the labels of the products available in your country for specific information on approved indications.


Oral LD50, rat, acute*: 178 - 615 mg/kg
Dermal LD50, rat, acute*: 271 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) set for animal tissues (e.g. beef, mutton pork or chicken)*:

  • CODEX: No
  • EU: Yes (Annexes II & III)
  • USA: No (Withdrawn)
  • AUS: Yes

* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock. MRL's for animal tissues may be set also for agricultural pesticides that are not approved for use on animals but are used on commodities fed to animals. A MRL may be also set in the form of an IMPORT TOLERANCE for active ingredients not approved in a particular country but approved for imported animal commodities.

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

Learn more about fenthion safety (poisoning, intoxication, overdose, antidote, symptoms, etc.).

General safety information for antiparasitics is available in specific articles in this site (click to visit):


Never use agricultural or hygiene products with this or any other active ingredient on livestock or pets, even if there are veterinary products with this same active ingredient approved for use on animals. The formulations for agricultural or hygiene use are different and may be toxic for livestock or pets.

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


Decade of introduction: 1960
Introduced by: BAYER
Some original brands: TIGUVON, SPOTTON
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Yes, low and declining, as all organophosphates
Use in HORSES: No
Use in
DOGS and CATS: Yes, scarce

Main delivery forms: 

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


In livestock: Yes, as for all organophosphates: very frequent worldwide in such species as cattle ticks (Boophilus spp), horn flies (Haematobia irritans), sheep lice (Damalinia ovis), poultry mites (Dermanyssus gallinae), houseflies (Musca domestica), mosquitoes
In pets: Yes, quite frequent worldwide in dog and cat fleas (Ctenocephalides spp)

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


Fenthion is a classical, veteran parasiticide belonging to the organophosphates.

Fenthion was the first ectoparasiticide that could be administered as a spot-on to both livestock and pets, but not in single-dose vials or pipettes as most modern spot-ons for pets, but as simple drops.

It is still used in some countries in livestock and pets, mainly as a ready-to-use pour-on and spot-on against various flies.

Nevertheless, there is a clear trend to replace all organophosphates, including fenthion, with less toxic compounds.

Efficacy of fenthion

As most organophosphates fenthion is a broad-spectrum insecticide, acaricide and larvicide. Fenthion is particularly effective against various livestock myiases such cattle grubs (Hypoderma spp), bot flies (e.g. Dermatobia hominis) and against various biting flies of livestock.

However, resistance of important veterinary parasites to all organophosphates, including fenthion is widespread, especially in cattle ticks (Boophilus spp), horn flies (Haematobia irritans), sheep lice (Damalinia ovis), poultry mites (Dermanyssus gallinae), mosquitoes, dog and cat fleas (Ctenocephalides spp) and houseflies (Musca domestica). As a consequence, products with this active ingredient may not achieve the expected efficacy in many places. The same applies to all other organophosphates. This is also a reason for their progressive replacement with newer active active ingredients with a different mode of action.

Pharmacokinetics of fenthion

Percutaneous absorption (i.e. through the skin) of topically administered fenthion depends on the animal species, the administered dose, and the extension of the treated body surface. Fenthion is easier absorbed through the skin or after inhalation than other organophosphates.

Animals treated topically can also ingest fenthion through licking and grooming. Absorbed fenthion is partly metabolized, and excreted through urine and feces in about 3 days. Being quite lipophilic it is partly deposited in body fat and is subsequently released back to the bloodstream to be metabolized.

Mechanism of action of fenthion

As all organophosphates insecticides, fenthion acts on the nervous system of the parasites (but also of mammals, birds, fish and many organisms!) as inhibitor of acetylcholinesterase (also known as AchE), an enzyme that hydrolyzes acetylcholine (Ach). Ach is a molecule involved in the transmission of nervous signals from nerves to muscles (so-called neuromuscular junctions) and between neurons in the brain (so-called cholinergic brain synapses).

AchE's role is to terminate the transmission of nervous signals where Ach is the neurotransmitter (there are several other neurotrasmitters). By inhibiting the activity of AchE, carbamates prevent the termination of those nervous signals, i.e. the neurons remain in constant activity and excitation, massively disturbing the normal movements of the parasites. The bottom line for the parasites is that they are paralyzed and die more or less quickly. Organophosphates bind irreversibly to AchE, in contrast with carbamates, another chemical class of parasiticides, which bind reversibly to AchE.

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