Methomyl is an insecticidal active ingredient used in livestock premises against flies. It is also used against agricultural and household pests. It belongs to the chemical class of the carbamates.

Common name: METHOMYL

Type: Pesticide
Chemical class: carbamate


Molecular structure of METHOMYL






Type of action: non systemic contact and oral insecticide
Main veterinary parasites controlledhouseflies and other filth & nuisance flies

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*:  17-24 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.

MRL (maximum residue limit): Not applicable: not approved for livestock
Withholding periods for meat, milk, eggs: Not applicable: not approved for livestock


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: 1960
Introduced by: DUPONT
Some original brands: GOLDEN MALRIN
Patent: Expired (Particular formulations may be still patent-protected)

Use on
Main delivery forms:

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


On livestock: Yes, worldwide, reported on houseflies (Musca domestica), cross-resistance with organophosphates

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


Methomyl is a veteran carbamate used in several fly baits for livestock operations. It is also used in agriculture.

It acts mainly as an oral insecticide for flies, i.e. it does not control bloodsucking flies such as stable flies or horn flies.

However, resistance of houseflies and other pests to all carbamates, including methomyl, is widespread. As a consequence, products with this active ingredient may not achieve the expected efficacy in many places. The same applies to all other carbamates. This is also a reason for their progressive replacement with newer active ingredients with a different mode of action.

Notice. As a general rule this site does not provide information about off-label uses of antiparasitic active ingredients. In most countries veterinary doctors can prescribe a veterinary medicine (also a parasiticide) for indications that are not included in its label. This is often the case for minor species  (e.g. rabbits, guinea pigs, exotic mammals and birds, reptiles, etc.) and orphan diseases  (also parasites) that are not investigated by pharmaceutical companies for whatever reasons.

Mechanism of action of methomyl

As all carbamate insecticides, methomyl acts on the nervous system of the parasites (but also of mammals, birds, fish and many organisms!) as inhibitor of acetylcholinesterase (also known as AchE), an enzyme that hydrolyzes acetylcholine (Ach). Ach is a molecule involved in the transmission of nervous signals from nerves to muscles (so-called neuromuscular junctions) and between neurons in the brain (so-called cholinergic brain synapses).

AchE's role is to terminate the transmission of nervous signals where Ach is the neurotransmitter (there are several other neurotrasmitters). By inhibiting the activity of AchE, carbamates prevent the termination of those nervous signals, i.e. the neurons remain in constant activity and excitation, massively disturbing the normal movements of the parasites. The bottom line for the parasites is that they are paralyzed and die more or less quickly. Carbamates bind reversibly to AchE, in contrast with organophosphates, another chemical class of parasiticides, which bind irreversibly.