Eprinomectin is a broad-spectrum antiparasitic active ingredient used in veterinary medicine in cats and livestock against external parasites (lice, mites, etc.) as well as against internal parasites (e.g. roundworms). It is not used against agricultural and household pests. It belongs to the chemical class of the macrocyclic lactones.


Type: veterinary medicine
Chemical class: macrocyclic lactone


Molecular structure of EPRINOMECTIN








Type of action: Systemic & contact broad-spectrum ectoparasiticide and endoparasiticide.
Main veterinary parasites controlled: Gastrointestinal and pulmonary roundworms, lice, mites, horn flies; myiasis by screwworms, bot flies, warble flies, 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 eprinomectin used in domestic animals.


Oral LD50, rat, acute*:  55 mg/kg
Dermal LD50, rat, acute*: not found
* 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: 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. Eprinomectin is usually allowed for use on dairy cattle.

Learn more about eprinomectin safety.

General safety information for antiparasitics is available in specific articles in this site (click to visit):


Never use products for livestock on dogs and cats unless they are explicitly approved for both livestock and pets. Pets may not tolerate livestock formulations.

It is obvious that veterinary products are not intended for and should never be used on humans!!!


Decade of introduction: 1990
Introduced by: MERCK SHARP & DOHME (→ MERIAL)
Patent: Expired (particular formulations may be still patent-protected)

Use in LIVESTOCK: Moderate in cattle, sheep & goats

Use in
DOGS and CATS: YES, rather scarce
Main delivery forms: 

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


In livestock: Yes, cross-resistance with ivermectin, very frequent in gastrointestinal roundworms of sheep, goats and cattle. There are a few reports on cattle tick (Boophilus microplus) resistance to ivermectin likely to show cross-resistance to eprinomectin.

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


Eprinomectin is the "ivermectin" for dairy cattle, because for years it was the only macrocyclic lactone approved in most countries for use on dairy cows whose milk is intended for human consumption. In the meantime other macrocycli lactones (e.g. moxidectin, doramectin) have been also approved for use on dairy cows in some countries. Eprinomectin has also been approved for use in shee and goats in some countries.

During about two decades the 0.5% pour-on (EPRINEX) for cattle was the only formulation available with eprinomectin. Only in 2014 MERIAL introduced LONGRANGE, a long-acting injectable for cattle containing 5% eprinomectin. Interestingly, for many years MERIAL marketed a long-acting 3.15% ivermectin injectable (IVOMEC GOLD) for cattle in numerous countries (mainly Latin America), but not in the USA or the EU. And in 2017 MERIAL introduced EPRINEX MULTI, a 0.5% pour-on approved for use in cattle, sheep & goats.

In 2014 MERIAL introduced BROADLINE, a spot-on for cats containing a mixture of eprinomectin, fipronil, praziquantel and methoprene., the first eprinomectin-based product approved for cats.

Spectrum of activity and efficacy of eprinomectin are comparable to those of ivermectin. However only the pour-on formulation is approved for dairy cows, not the extended-release injectable.

Efficacy of eprinomectin

In cattle eprinomectin is effective against the major parasitic roundworms: gastrointestinal (e.g. Haemonchus spp, Cooperia spp, Ostertagia spp, Trichostrongylus spp) and pulmonary (e.g. Dictyocaulus spp). It is also effective againts several ectoparasites such as mites and lice species, and against numerous myiases (e.g. those caused by screwworm flies, bot flies and warble flies).

strong>However, at the usual therapeutic dose, eprinomectin products are ineffective against tapeworms and flukes, fleas, mosquitoes, fleas, horse and deer flies and stable flies and several other flying insects. The eprinomectin pour-on provides some control of horn flies and cattle ticks (Boophilus spp). But other ticks (e.g. Amblyomma spp, Hyalomma spp Dermacentor spp, Ixodes spp, Rhipicephalus spp) are not controlled at all by eprinomectin products.

Pharmacokinetics of eprinomectin

After topical administration (pour-on) only about 46% of the administered dose is absorbed through the skin, the rest remains outside and breaks down slowly or is washed away. The halftime for absorption in dairy cows is about 0.95 days. Most of the administered dose is absorbed in the first 7 to 10 days after treatment, but absorption continues until 17 to 21 days. Peak plasma concentrations are reached 2 to 5 days after treatment.

After treatment with the extended-release injectable a polymeric PLGA [poly(lactic-co-glycolic) acid] matrix is formed in the subcutanous tissue. The active ingredient is slowly released from the matrix into the bloodstream maintaing effective parasiticidal blood levels for months.

Once absorbed into blood eprinomectin is poorly metabolized in the bovine's organism. It is well distributed throughout the host's body and the maximum concentrations are found in liver and kidneys. Metabolism after treatment with the extended-release injectable is also scarce.

After pour-on administration, eprinomectin does not appear in milk and this is why it was the only macrocyclic lactone approved for use in dairy cows (only the pour-on formulation!).

Eprinomectin is excreted mainly through the feces, only a very small portion through the urine.

Mechanism of action of eprinomectin

As all macrocyclic lactones eprinomectin acts as agonist of the GABA (gamma-aminobutyric acid) neurotransmitter in nerve cells and also binds to glutamate-gated chloride channels in nerve and muscle cells of invertebrates. In both cases it blocks the transmission of neuronal signals of the parasites, which are either paralyzed and expelled out of the body, or they starve. It also affects the reproduction of some parasites by diminishing oviposition or inducing an abnormal oogenesis.

In mammals the GABA receptors occur only in the central nervous system (CNS), i.e. in the brain and the spinal chord. But mammals have a so-called blood-brain barrier that prevents microscopic objects and large molecules to get into the brain. Consequently macrocyclic lactones are much less toxic to mammals than to the parasites that do not have such a barrier, which allows quite high safety margins for use on livestock and pets. A notable exception to this are those dogs that carry the MDR-1 mutation.

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