S-methoprene is the biologically active enantiomer in the racemic mixture of methoprene and constitutes 50% of methoprene.

WHO Acute Hazard classification of pesticides: U, unlikely to present acute hazard in normal use

Mechanism of action of Methoprene

Methoprene is a Juvenile Hormone Analogue (JHA) acting as an insect development inhibitor (also called Insect Growth Regulator = IGR) effective against numerous insect species, but with no effect whatsoever on ticks or mites. It only interferes with the molt from pupae to adults, i.e. it does not kill insect larvae and consequently it cannot be used against various myiases.

Methoprene and other JHAs suppress or stimulate the expression of various genes involved in insect metamorphosis otherwise regulated by natural juvenile hormone. Depending on which gene is affected different biochemical and cellular effects will result. The bottom line is that development is disturbed and interrupted. Since these biochemical processes do not occur in vertebrates, methoprene and most other JHAs can be considered as rather safe for humans, pets and livestock.

In livestock methoprene is only used in a few feed additives for cattle against horn flies, sometimes used also in horses. Fed to cattle a large portion goes unchanged through the gut (feed-though), is excreted with the feces and acts upon the larvae of flies developing in the manure.

In dogs and cats it is mostly used in combination with flea adulticides (e.g. fipronil, etofenprox, permethrin, propoxur, etc) in numerous spot-ons (= squeeze-on = pipettes), as well as in shampoos, soaps, sprays, etc. It is used mainly to add efficacy against the developmental stages of the fleas (eggs, larvae), since most flea adulticides have an insufficient impact on the developmental stages in the pets' environment. From the safety point of view, the adulticides in such products are much more capable of causing adverse drug reactions.

Acute Toxicity and Tolerance of Methoprene

  • LD50 acute, rats, oral >34600 mg/kg
  • LD50 acute, dogs, oral: 5000 - 10000 mg/kg
  • LD50 acute, rabbits, dermal 3038 - 10250 mg/kg
  • LD50 8 days diet, chickens >4640 mg/kg/8 days
  • Two year feeding trials in rats: >5,000 mg/kg/day in the diet
  • Methoprene is not carcinogenic, teratogenic or mutagenic
  • It is non-irritating to skin and eyes.
  • Dogs, cats and livestock tolerate methoprene very well
  • Usual therapeutic dose (topical) in dogs and cats is 5 - 40 mg/kg
  • Usual therapeutic does (oral) in cattle is 75 mg/100 kg bw/month

Toxic Symptoms, Side Effects, Adverse Drug Reactions (ADRs) and Warnings

  • As a general rule, intoxications with methoprene are very infrequent due to its low toxicity, the high safety margin and the excellent tolerance in pets and livestock.
  • Since methoprene is metabolized and excreted though the liver and the kidneys, assessment of renal and hepatic functions may be indicated in case of suspected poisoning.
  • In pets, since it is mainly used in mixtures with other insecticides, observed ADRs are more likely to be caused by other active or inert ingredients than by methoprene.

Antidote and Treatment of Methoprene Intoxication

  • There is no antidote for methoprene poisoning.
  • Treatment consists in preventing further exposure together with supportive and symptomatic measures.

Pharmacokinetics of Methoprene

  • Topically administered methoprene is poorly absorbed through the skin. After oral administration to cattle most of the administered dose is excreted unchanged through the feces. The rest is absorbed, metabolized and excreted through urine, feces and expired breath. About 20% of the administered dose is excreted through urine in the form of various metabolites.

Environmental Toxicity of Methoprene

  • Methoprene is moderately toxic to fish, and toxic to amphibians an reptiles. Its low solubility in water and its rapid degradation reduce the risk of exposure of these organisms to harmful concentrations of methoprene in water.
  • Methoprene is highly toxic to some aquatic arthropods such as insects and crustaceans.
  • Methoprene is not persistent in soils: it is broken down quickly by microbial degradation. It is also quickly degraded when exposed to sunlight. Half-life in sandy-loam was ~10 days.
  • Methoprene does not cumulate in the environment or in the food chain.
  • Correctly used in dogs, cats and livestock methoprene is unlikely to be detrimental for the environment.

Additional information

Click here for a list and overview of all safety summaries of antiparasitic active ingredients in this site.

  • Methoprene is an insect growth regulator belonging to the group of the Juvenile Hormone Analogues.
  • Methoprene is not used in human medicines.
  • Methoprene is moderately used in crop pesticides.
  • Methoprene is also used in public and domestic hygiene, mainly against mosquito larvae in water.
  • Click here for General safety of antiparasitics for domestic animals.
  • Click here for General safety of antiparasitics for humans.
  • Click here for General safety of antiparasitics for the environment.
  • Clik here for technical and commercial information on methoprene.


If you intend to use a veterinary drug containing this active ingredient you must carefully read and follow the safety instructions in the product label.  Always ask your veterinary doctor, or pharmacist, or contact the manufacturer. Be aware that the safety instructions for the same veterinary medicine may vary from country to country.

The information in this page must not be confused with the Materials and Safety Datasheets (MSDS) officially issued by manufacturers for active ingredients and many other chemicals. MSDSs target safety during manufacturing, transport, storage and handling of such materials. This safety summary is a complement to the information on product labels and MSDS.

The toxicity of an active ingredient must not be confused with the toxicity of finished products, in this case parasiticidal drugs or pesticides. Finished products contain one or more active ingredients, but also other ingredients that can be relevant from the safety point of view.

All information in this site is made available in good faith and following a reasonable effort to ensure its correctness and actuality. Nevertheless, no this regarding guarantee is given, and any liability on its accuracy, integrity, sufficiency, actuality and opportunity is denied. Liability is also denied for any possible damage or harm to persons, animals or any other goods that could follow the transmission or use of the information, data or recommendations in this site by any site visitor or third parties.