Chlorpyrifos 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 and household pests. It belongs to the chemical class of the organophosphates.


Type: pesticide
Chemical class: organophosphate


Molecular structure of CHLORPYRIFOS






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. 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.


Click here to view the article in this site with the most common dosing recommendations for chlorpyrifos used in domestic animals.


Oral LD50, rat, acute*: 135 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: 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. 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 chlorpyrifos 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: DOW CHEMICAL
Some original brands: DURSBAN
Patent: Expired (particular formulations may be still patent-protected)

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

Main delivery forms

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


  • 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).

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


Chlorpyrifos is a classical, veteran parasiticide belonging to the organophosphates. It has been used in livestock and pets. It was banned in the USA in 2018.

In livestock it is still used in some countries on cattle, sheep and pig in concentrates for dipping, spraying, and dusting, in ready-to-use pour-ons and dressings and in insecticide-impregnated ear-tags.

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

There are also numerous mixtures, mainly with synthetic pyrethroids (e.g. cypermethrin, permethrin).

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

Efficacy of chlorpyrifos

As most organophosphates chlorpyrifos is a broad-spectrum insecticide, acaricide and larvicide. Chlorpyrifos is a "generalist" pesticide, with good average efficacy against most external parasites, but not outstanding against a particular one.

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

Percutaneous absorption (i.e. through the skin) of topically administered chlorpyrifos depends on the animal species, the administered dose, and the extension of the treated body surface. Animals treated topically can ingest chlorpyrifos through licking and grooming. Orally administered chlorpyrifos is partly excreted unchanged through the feces.

Once absorbed into blood it is quickly metabolized in the liver to less toxic compounds and rapidly excreted through urine. Excretion half-life is about 2 to 3 days. Administered as a spray to cows it was detected in their milk for 4 days following treatment.

Mechanism of action of chlorpyrifos

As all organophosphates insecticides chlorpyrifos 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.