Company: DURVET

FORMULATION: spot-on for topical administration.

ACTIVE INGREDIENT(S): permethrin: 45% = 450 g/L

CHEMICAL CLASS of the active ingredient(s): synthetic pyrethroid


PARASITES CONTROLLED (spectrum of activity)


  • Apply the content of one tube (0.3 oz = 9 ml) to the diffrent parts of the animal's body according to the label recommendations.
  • Do not reapply for 14 days


  • LD50 (acute oral) in rats: for permethrin 250 - 4150 mg/kg depending on the vehicle
  • LD50 (acute dermal) in rats: for permethrin >4920 mg/kg
  • Estimated hazard class according to the WHO: III, slightly hazardous

Suspected poisoning? Read the article on permethrin safety in this site.

Withholding periods (=withdrawal times) in days for meat & milk (country-specific differences may apply: read the product label)

  • Do not use on horses intended for human consumption.

WARNING !!!: Never use on humans, dogs or cats. Permethrin is particularly toxic to cats!

Permethrin and all other 2nd-generation synthetic pyrethroids (e.g. cyhalothrin, cypermethrin, deltamethrin, etc.) are irritant to the eyes and the skin, both of humans and livestock. The inert ingredients in the formulation may worsen this side effect. Irritation can be particularly problematic for dairy cows because it can significantly hinder handling for milking.

You may be interested in the following articles in this site dealing with the general safety of veterinary products:


Risk of resistance? YES, resistance of horn flies & houseflies to synthetic pyrethroids (incl. permethrin) is widespread worldwide, and can be very high. Cases of resistance of black flies, stable flies and little house flies to synthetic pyrethroids (incl. permethrin) have also been reported, but prevalence is usually low.

This means that if this product does not achieve the expected efficacy against the mentioned parasites, it can be due to resistance and not to incorrect use, which is usually the most frequent cause of product failure.

Alternative chemical classes/active ingredients to prevent resistance of external parasites through product rotation:

These alternative products may not be available in all countries or may not be effective against all the concerned parasites.

Learn more about resistance and how it develops.


Are the active ingredients of this product ORIGINAL* or GENERICS**?


*Meaning that they are still patent protected and generics are not yet available
**Meaning that they have lost patent protection and may be acquired from manufacturers of generic active ingredients other than the holder of the original patent.

COUNTRIES where this brand/product is marketed: USA
GENERIC BRANDS available? YES, numerous, perhaps not in this particular presentation. This brand itself contains generic permethrin.

Click here to learn more about GENERIC vs. ORIGINAL drugs.

For an overview on the most used antiparasitic pour-on brands click here.


FLYRID PLUS from DURVET is one of numerous insecticides for horses or livestock containing synthetic pyrethroids for the control of flies, lice and other external parasites. Worldwide there are hundreds if not thousands of such products, mostly as pour-ons. Besides permethrin, numerous other synthetic pyrethroids are used in such products, e.g. cyhalothrin, cypermethrin, deltamethrin, etc. They all have a similar spectrum of activity and a comparable safety profile.

All synthetic pyrethroids are veteran pesticides developed in the 1970s-1980s and are basically contact insecticides. This means that when the parasite comes in contact with it (e.g., during the blood meal, after landing on a treated host, etc), the active ingredient that impregnates the host's hair coat penetrates through the cuticle of the parasite (the "skin" of insects and other arthropods) into its organism and disturbs essential biological processes in the parasite's body, in this case its nervous system.

After administration to livestock or other animals, synthetic pyrethroids do not have a systemic mode of action, i.e. they are not transmitted to the parasite through the blood or the host. Topically administered synthetic pyrethroids are very poorly absorbed through the skin of the hosts, and what is absorbed is quickly broken down and/or excreted. Consequently the concentration reached in the blood is too low to kill blood-sucking parasites. But this is why they are considered rather safe for mammals, both humans and livestock (cats are an exception: pyrethroids are toxic to them!) and why they leave rather low residues in meat and milk.

Control of susceptible (i.e. non-resistant) horn flies (mainly a cattle parasite, but may occasionally affect horses as well) is usually good, because they spend a lot of time on cattle and thus are exposed to the insecticide for a long period of time. Lice are also exposed to insecticides for a long period of time because they never leave the host. Stable flies, horse fliesblack flies and mosquitoes may bite the treated animal anywhere in its body and remain attached and thus exposed to the insecticide only during their blood meals that last a few seconds or minutes, which is often too short to kill them.

All synthetic pyrethroids have a certain repellent effect, mainly on mosquitoes but it lasts usually only a few days, if at all.

It is useful to know that the active ingredients of many synthetic pyrethroids consist in a mixture of various optical isomers, typically those called "cis", and those called "trans". Permethrin has 4 isomers, 2 cis, and 2 trans. Manufacturers of active ingredients usually supply the raw material in standard mixtures, for permethrin typically in a 25/75 or 40/60 cis/trans ratio. It happens that the efficacy against parasites and the mammalian toxicity of these isomers are significantly different. Typically cis isomers are more effective insecticides but also more toxic to mammals. Obviously a cis/trans 40/60 mixture is more potent than a cis/trans 25/75 mixture. Qualities with a higher cis content are usually also more expensive. And the higher the percentage of the most active isomer, the lower the rate that is required for achieving the same efficacy. This particular product contains 50/50 cis/trans permethrin.


This article IS NOT A PRODUCT LABEL. It offers complementary information that may be useful to veterinary professionals and users that are not familiar with veterinary antiparasitics. 

Information offered in this article has been extracted from publications issued by manufacturers, government agencies (e.g. EMEA, FDA, USDA, etc.) or in the scientific literature. No guarantee is given on its accuracy, integrity, sufficiency, actuality and opportunity, and any liability is denied. Read the site's DISCLAIMER.

In case of doubt contact the manufacturer or a veterinary professional.

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