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)


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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



Oral LD50, rat, acute*: 430-630 mg/kg


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



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






Decade of introduction: 1960
Introduced by: BAYER
Some original brands: NEGUVON

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: 



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.


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




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