Cyromazine is an antiparasitic active ingredient used in veterinary medicine in livestock and horses against a few external parasites (blowfly strike, houseflies, etc.). It is also used against agricultural pests. It is a so-called Insect Growth Regulator (IGR) belonging to the chemical class of the triazines.

Common name: CYROMAZINE

Type: pesticide
Chemical class: triazine, insect development inhibitor


Molecular structure of CYROMAZINE







Type of action: contact and oral larvicide
Main veterinary parasites controlled: larvae (maggots) houseflies, blowflies, lice, etc.

Efficacy against a specific parasite depends on the delivery form and on the dose administered. 

Click here for general information on features and characteristics of PARASITICIDES.


Cyromatine is an insect development inhibitor quite specific and effective against dipteran larvae, but ineffective against other external parasites (lice, mites, ticks, etc.).

It is used a lot as a blowfly strike preventative in sheep (spraying and jetting, pour-on) and as a feed-thru larvicide (houseflies, filth & nuisance flies) in poultry (feed additives).

There are also e few feed-thru larvicides for horses in some countries.

It is also abundantly used for off-animal treatment of livestock facilities against any kind of flies. It is not used  in pets.

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

Dosing recommendations for CYROMAZINE
Delivery Parasites Dose (against cyromazine-susceptible parasites)
Spray, jetting Blowfly strike 1000 ppm (= mg/L); animals with at least 6 weeks wool
Pour-on Blowfly strike ~50-200 mg/kg, dep. on wool length
Oral (feed-thru) Houseflies & filth flies 300 mg/animal/day or 600 mg/animal every other day during the fly season, mixed in daily feed ratio.
Oral (feed-thru) Houseflies & filth flies 5 ppm (=mg/kg) in feed during 4-6 weeks

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

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 topical pour-ons and spot-ons. These inert ingredients can significantly affect the pharmacokinetic behavior (e.g. absorption through the skin, 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.


Oral LD50, rat, acute*: 3387 mg/kg
Dermal LD50, rat, acute*: >3100 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 (either beef, mutton pork or chicken)*:

  • CODEX: Yes
  • EU: Yes
  • 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 cyromazine safety.

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: 1980
Some original brands: VETRAZIN, LARVADEX, NEPOREX
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Yes, moderate

Use in HORSES: Yes, scarce
Use in
Main delivery forms

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


In livestock: YES

Tolerance (i.e. very low resistance) has been reported in houseflies (Musca domestica), but it seems to be reversible when selection pressure stops.

In 2012 a first field strain of Lucilia cuprina blowflies tolerant to cyromazine has been reported in Australia more than 30 years after its introduction. It shows a low resistance factor of 3 and cross resistance with dicyclanil.

In 2020 resistance has been reported in two field strains of Lucilia cuprina in Australia reaching in vitro resistant factors of 13 to 25 when compared with suscetible dicyclanil and cyromazine strains, respectively. In-vivo studies showed that protection against re-infestation was reduced by 69% to 78% when compared with the claims of the manufacturer for dicyclanil. These strains also showed a 55% and 33% reduction of the length of protection after treatment with commercial products based on cyromazine and ivermectin, respectively.

Learn more about parasite resistance and how it develops.


Cyromazine belongs chemically to the triazines, a chemical class with several herbicidal compounds. It is a rather specific insect development inhibitor effective against larvae of most dipteran insects (flies, mosquitoes, etc.), fleas and some beetles.

It is available for sheep as concentrates for dipping and spraying and as pour-on por blowfly strike prevention, as a feed additive for poultry to control houseflies in the manure, and as a concentrate for manure and environmental treatment against houseflies and other nuisance flies.

In the past there were a few products for on-animal and off-animal flea control in dogs but they have been mostly withdrawn.

Pharmacokinetics of cyromazine

Topically administered cyromazine is poorly absorbed through the skin. Cyromazine is quite soluble in water, in contrast with many ectoparasiticides that are lipophilic. After oral administration to poultry 99% of the administered dose is excreted unchanged in the feces. Only about 0.6% is deposited in eggs and very few in chicken tissues. Both in mammals and poultry most ingested cyromazine is absorbed into blood. A small portion is metabolized in the liver and the largest portion is excreted through urine. Excretion is rather fast: about 95% of the ingested dose is excreted within 24 hours after administration.

In treated poultry, since cyromazine excreted with the urine is mixed with the feces in the cloaca, the whole manure gets uniformly treated, which allows a rather effective control of fly maggots. In mammals, ingested cyromazine does not end in the feces but in the urine, and consequently it cannot be used as an effective feed-through larvicide for manure fly control in most situations.

Mechanism of action of cyromazine

The mechanism of action of cyromazine has not been completely elucidated. It is certainly not a Chitin Synthesis Inhibitor like other development inhibitors such as the benzoylureas. It nevertheless acts upon the molting process, probably interfering with the process of chitin deposition.

Curiously, the massive use of cyromazine against blowfly strike in Australia, New Zealand, UK and Ireland for more than 40 years has not resulted in development of strong resistance by these flies until recently. This although blowflies have developed resistance to other larvicides such as organophosphates and benzoylureas in not much longer than 10 years after ther introduction. The reasons for this are still not elucidated.

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