Permethrin is a synthetic pyrethroid highly effective against many external livestock and pet parasites (flies, ticks, mites, lice, fleas, mosquitoes, etc.). It is completely ineffective against internal parasites.

As all synthetic pyrethroids, permethrin products are always used for topical administration, either as concentrates for dipping or spraying, or in ready-to-use products such as pour-ons, dressings, ear-tags for cattle, sheep, swine and horses, and spot-ons or low-cost topicals for dogs such as shampoos, soaps, sprays, etc. Permethrin is used massively in pets worldwide, with thousands of different brands. Use in livestock is lower because cypermethrin is often preferred in many countries. It is also used in crop protection and household pesticides.

Permethrin (as all synthetic pyrethroids) acts by contact (i.e. it is not systemic) and is effective against most external livestock and pet parasites (flies, ticks, mites, lice, fleas, mosquitoes, etc.) but is completely ineffective against internal parasites.

However, resistance to all synthetic pyrethroids is already very frequent worldwide and can be extremely high, particularly in cattle ticks, horn & buffalo flies, houseflies, red fowl mites, mosquitoes, sheep lice, etc.

Dosing and efficacy of permethrin products depend a lot on its cis:trans isomeric composition (e.g. 25:75, 40:60, 80:20). The higher the cis-isomer content the higher the efficacy at the same dose, or the lower the dose required to achieve the same control than with the low-cis product. Unfortunately many manufacturers do not indicate the cis:trans composition in the product labels, which may create confusion.

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

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In most finished products, efficacy and safety depend not only on the amount of active ingredient(s) but also on their formulations (i.e. the type and amount of so-called inert ingredients), particularly in injectables and in topical pour-ons and spot-ons. These inert ingredients can significantly affect the pharmacokinetic behavior (e.g. absorption through the skin or from the injection site, distribution within the body, spreading throughout the body surface, etc). Generic products usually contain the same amount of active ingredient(s) as the original product, but often in quite different formulations. In many cases, the curative (therapeutic) efficacy of the different formulations is quite comparable, but the protective (prophylactic) efficacy that determines the length of protection against re-infestations may be rather different.

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 parasite. Check the labels of the products available in your country for specific information on approved indications.

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