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


Type: pesticide
Chemical class: organophosphate


Molecular structure of TRICHLORFON = METRIFONATE





Type of action: Broad spectrum ectoparasiticide: insecticide, acaricide, tickicide, louisicide, larvicide, as well as endoparasiticide
Main veterinary parasites controlledflies, lice, fleas, mosquitoes, ticks, mites, fly larvae, etc; several gastrointestinal roundworms (nematodes)

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

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


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.

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

Trichlorfon is a broad-spectrum organophosphate highly effective against many external livestock and pet parasites (flies, ticks, mites, lice, fleas, mosquitoes, etc.) but is completely ineffective against internal parasites when administered topically (i.e. on the skin).

As most organophosphates, thrichlorfon products are used mainly for topical administration, either as concentrates for for dipping or spraying, or in a few ready-to-use products such as dressings, dusts, and pour-ons for livestock. It is also used in a few low-cost topicals for dogs such as shampoos, soaps, sprays. However, trichlorfon is one of the few organophosphates that is still used as an anthelmintic drench for livestock in a few countries, as well as in oral pastes for horses.

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

Dosing recommendations for TRICHLORFON
Delivery Parasites Dose (against trichlorfon-susceptible parasites)
Bath Fleas, ticks, lice, mites, etc. 1500 ppm (=mg/L) in the wash
Delivery Parasites Dose (against trichlorfon-susceptible parasites)
Dip Ticksflies, mites, lice 1500 ppm (=mg/L) in the wash
Oral Cattle grubs 75-100 mg/kg
Oral Gastrointestinal roundworms 50-75 mg/kg
Pour-on Flies, lice 2-10 mg/kg
Spot-on Cattle grubs 5-10 mg/kg
Delivery Parasites  Dose (against trichlorfon-susceptible parasites)
Dip, spray Lice, mites 1500 ppm (=mg/L) in the wash
Oral Gastrointestinal roundworms 50-75 mg/kg
Oral Nasal bots (Oestrus ovis) 75 mg/kg
Pour-on Lice 2-10 mg/kg
Delivery Parasites  Dose (against trichlorfon-susceptible parasites)
Oral Gastrointestinal roundworms 50-75 mg/kg
Oral Stephanurus dentatus 63-65 mg/kg
Pour-on Lice 2-10 mg/kg
Spray Mites 19000 (!!!) ppm (=mg/L) in the wash
Spray Fleas 1500 ppm (=mg/L) in the wash
Delivery Parasites  Dose (against trichlorfon-susceptible parasites)
Oral Gastrointestinal roundworms 20-40 mg/kg
Oral Gasterophilus spp 10-40 mg/kg

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Oral LD50, rat, acute*: 430-630 mg/kg
Dermal LD50, rat, acute*: >2000 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
  • USA: No
  • 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 trichlorfon 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: 1950
Introduced by: BAYER
Some original brands: NEGUVON
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Yes, low and declining, as all organophosphates
Use in HORSES: Yes, scarce and declining as all organophosphates
Use in
DOGS and CATS: Yes, low and declining, as all organophosphates

Main delivery forms

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


In livestock & horses: 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)

Learn more about parasite resistance and how it develops.


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

Trichlorfon 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.

In livestock it is still used in some countries in cattle, sheep and pig in concentrates for dipping and spraying, in ready-to-use pour-ons and dressings and in drenches against several gastrointestinal roundworms.

Trichlorfon is one of the very few organophosphates still used in some countries against gastrointestinal roundworms of sheep and goats that have become resistant to modern anthelmintics (benzimidazoles, macrocyclic lactones, imidazothiazoles). For this puropose it is administered orally. It has to be done very accurately because the safety margin is extremely low and slight overdosing can easily cause toxic side effects.

In dogs and cats it is still used in shampoos, soaps, baths, sprays, powders and the like, but its usage has strongly declined after the introduction of more modern and safer flea and tick control spot-ons (= pipettes) and tablets.

It is often used in mixtures, mainly with synthetic pyrethroids (e.g. cypermethrin, permethrin).

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

Efficacy of trichlorfon

As most organophosphates trichlorphon is a broad-spectrum insecticide, acaricide and larvicide. Trichlorfon is particularly effective against various livestock myiases such as maggots from warble flies (Hypoderma spp), human bot flies (e.g. Dermatobia hominis) and screwworm flies.

However, resistance of important veterinary parasites to all organophosphates, including trichlorfon 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 trichlorfon

Percutaneous absorption (i.e. through the skin) of topically administered trichlorfon depends on the animal species, the administered dose, and the extension of the treated body surface. Trichlorfon is easier absorbed through the skin or after inhalation than most other organophosphates. Animals treated topically can ingest trichlorfon through licking and grooming. However, once absorbed into blood it is very quickly metabolized in the liver to less toxic compounds and very rapidly excreted through urine. Interestingly, one of the main metabolites of trichlorfon is dichlorvos, which itself is unstable and is very quickly hydrolyzed. In cattle, maximum excretion in urine occurs 2 to 4 hours after administration. It is no more detectable in urine 4 days after administration.

Mechanism of action of trichlorfon

As all organophosphates insecticides, trichlorfon 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.

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

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