Phenothrin is an antiparasitic active ingredient used in veterinary medicine in pets against external parasites (lice, mites, fleas, fliesticks, etc.). It is also used against household pests. It belongs to the chemical class of the synthetic pyrethroids.

Common name: PHENOTHRIN

Type: pesticide
Chemical class: synthetic pyrethroid


Molecular structure of PHENOTHRIN







Type of action: Broad spectrum contact, non systemic ectoparasiticide: insecticide, acaricide, tickicide, louisicide.
Main veterinary parasites controlled: flies, ticks, mites, lice, fleas, mosquitoes, 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.


Oral LD50, rat, acute*:  >5000 mg/kg
Dermal LD50, rat, 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.

Phenothrin is toxic to cats!

Synthetic pyrethroids can be irritant for the skin and the eyes.

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

  • EU: Yes (Annex IIIA, provisional, limit of detection)
  • USA: No
  • AUS: No

* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock. An MRL for meat may be established also for agricultural pesticides that are not approved for use on animals but are used on commodities fed to animals. It may be also established 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 phenothrin safety.

General information on the safety of veterinary 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: 1970
Introduced by: SUMITOMO
Some original brands: SUMITHRIN, MULTICIDE
Patent: Expired (particular formulations may be still patent-protected)

Use in
DOGS: Yes, moderate

Main delivery forms:

Use in human medicine: Yes
Use in
public/domestic hygiene: Yes
Use in
agriculture: Yes
Generics available: 


In pets: Yes, quite frequent resistance to synthetic pyrethroids worldwide in dog and cat fleas (Ctenocephalides spp).

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


Phenothrin is a synthetic pyrethroid with a spectrum of activity similar to permethrin. It is moderately used on dogs, not at all on livestock. It is also used in agriculture as well as in public and domestic hygiene.

For dogs it is often used in mixtures with insect development inhibitors, etc. Some products contain also synergists.

Phenothrin is a mixture of various optic isomers with different insecticidal efficacy. Commercial products may contain different isomer mixtures. For most users, it often doesn't make any difference regarding efficacy, because if one product uses a mixture with more of the most effective isomers, it will be used at a lower concentration than a product using a mixture with less effective isomers.

Efficacy of phenothrin

Phenothrin is an ectoparasiticide, i.e. active only against external parasites such as flies, lice, mosquitoes, ticks and mites. It can be considered as a broad-spectrum generalist, i.e. quite good against almost all insects, ticks and mites, but not outstanding against a particular parasite. 

Phenothrin, as many other synthetic pyrethroids has a significant repellent effect on certain insects and ticks, which strongly depends on the delivery form and the dose administered.

However, resistance to phenothrin is widespread and can be very high in dog and cat fleas(Ctenocephalides spp). As a consequence, products with phenothrin may not achieve the expected efficacy. The same applies to all other synthetic pyrethroids (e.g. cypermethrin, deltamethrin, flumethrin, etc.). And this is true for whatever delivery.

Pharmacokinetics of phenothrin

Topically administered phenothrin remains mostly on the hair-coat of the treated animals and is very poorly absorbed through the skin. Phenothrin is not very resistant to sunlight, in contrast with newer synthetic pyrethroids (e.g. cypermethrin, deltamethrin, etc.), which shortens the residual effect (protection period).

Treated animals can ingest phenothrin through licking or grooming. Absorption to blood is low. The absorbed phenothrin is quickly metabolized in the liver to non-toxic metabolites that are excreted through urine and feces, depending on the isomers. Metablosim is done by a specific enzyme called glucuronidase. However, cats lack this enzyme and cannot metabolize synthetic pyrethroids properly. This is why most synthetic pyrethroids are toxic to cats.

Mechanism of action of phenothrin

Synthetic pyrethroids, including phenothrin, have a similar mode of action as organochlorines. They act on the membrane of nerve cells blocking the closure of the ion gates of the sodium channel during re-polarization. This strongly disrupts the transmission of nervous impulses. At low concentrations insects suffer from hyperactivity. At high concentrations they are paralyzed and die.

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