WHO Acute Hazard classification: not listed, it is not used as a crop pesticide.

Mechanism of action of Flumethrin

Synthetic pyrethroids, including flumethrin, have a similar mode of action as organochlorines. They act on the membrane of nerve cells blocking the closure of the ion gates of the sodium channel during re-polarization. This strongly disrupts the transmission of nervous impulses, causing spontaneous depolarization of the membranes or repetitive discharges. At low concentrations insects and other arthropods suffer from hyperactivity. At high concentrations they are paralyzed and die. Sensory and nervous cells are particularly sensitive.

Acute Toxicity and Tolerance of Flumethrin

  • LD50 acute, rats, p.o. 41 (oil vehicle) to 3859 (aqueous vehicle). The vehicle-dependant differences in the acute oral toxicity are typical for synthetic pyrethroids.
  • LD50 acute, rats, dermal, >2000 mg/kg
  • In toxicity studies, dogs (Beagles) were fed flumethrin mixed with food at 25, 50, 100 and 200 mg/kg feed during 13 weeks. At the end of the study all dogs showed skin lesions (thickened skin covered with hyperkeratotic material) at doses >50 mg/kg feed. The NOEL (No Observable Effect Level) was 25 mg/kg feed, equivalent to 0.88 mg/kg bw per day.
  • In cattle treated topically at 2x the therapeutic dose a few animals showed erythema (skin redness) at the application site and transient diarrhea.
  • In cattle, the pour-on formulation has a safety margin of 50. At doses of up to 10 mg/kg no adverse effects were recorded on skin, hair coat or hides. Five consecutive treatments at a dose 4 mg/kg in intervals of 3 to 4 days did not cause adverse effects.
  • Sheep and goats treated at 10x the therapeutic dose didn't show signs of intoxication. At 20x the therapeutic dose 50% of the sheep died, but it could not be excluded that this was due to the vehicle.
  • As a general rule, dogs and livestock (cattle, sheep, goats) tolerate flumethrin and most synthetic pyrethroids very well, since toxicity is about 1000x higher to insects and other arthropods than to mammals. But toxicity to mammals can be higher in case of sustained skin or inhalation exposure, or after direct contact with open wounds.
  • WARNING: cats are more susceptible to synthetic pyrethroids than dogs. Cats may not tolerate doses of flumethrin that are harmless for dogs. This is associated with glucuronidase deficiency in cats, the enzyme responsible for breaking down most synthetic pyrethroids in the organism in a process called glucuronidation. As a consequence, synthetic pyrethroids remain much longer in the cat's organism than in dogs or other mammals.

Toxic Symptoms caused by Flumethrin Poisoning

  • The primary symptoms of intoxication with flumethrin and other synthetic pyrethroids affect mainly the nervous and muscular systems.
  • Most frequent symptoms are:
    • Ataxia (uncoordinated movements)
    • Hyperreactivity (exaggerated reaction to stimuli)
    • Tremor (uncoordinated trembling or shaking movements)
    • Paresthesia (skin sensation of tingling, tickling, prickling)
    • Exhaustion (lethargy, fatigue)
    • Hypersalivation (drooling)
    • Vomit
    • Diarrhea
    • Urinary incontinence
  • Other symptoms after severe poisoning include: hyperthermia (fever) or hypothermia (too low body temperature), dyspnea (difficult breathing), disorientation, cramps or spasms (sudden, involuntary contractions of muscles or hollow organs).
  • Symptoms appear a few hours after exposure, but depend strongly on the formulation, the dose and the kind of contact (skin, inhalation, ingestion etc).
  • Sustained skin exposure can cause local dermatitis (skin irritation) with pruritus (itching) and erythema (red skin).
  • Mucous membranes are particularly sensitive to synthetic pyrethroids, e.g. in the nose and the respiratory system (coughing), in the eyes (conjunctivitis), genital organs, etc.
  • After excessive inhalation of synthetic pyrethroids patients can develop allergic sensitization with asthmatic symptoms. In extreme cases, sustained inhalation of high doses can cause respiratory paralysis and death.
  • As a general rule, young animals are more sensitive to overdosing and react stronger. 
  • A frequent administration error in dogs is partial administration to small dogs of spot-ons approved for large dogs.

Flumethrin Side Effects, Adverse Drug Reactions (ADRs) and Warnings

  • Do not administer flumethrin topically (spot-on, shampoos, soaps, sprays, etc.) in case of extended skin lesions: this can lead to an excessive absorption through the damaged skin.
  • Pour-ons containing flumethrin and other synthetic pyrethroids can be irritant for cattle. This can be particularly annoying when handling dairy cows for milking.
  • In small dogs paresthesia (skin sensation of tingling, tickling, prickling) can happen at the therapeutic dose, which usually disappears in 12 to 24 hours.
  • Toxic effects can be potentiated after simultaneous exposure to organophosphates or other synthetic pyrethroids.
  • Never use spot-ons or other products on cats that are approved only for dogs: synthetic pyrethroids can be toxic to cats.
  • Never use spot-ons for large dogs on small dogs. It happens that some users want to save money buying large spot-ons for treating smaller dogs twice or more times. The risk of overdosing is considerable, either due to erroneous calculations or to unskilled manipulation. Remaining product in opened spot-on vials can deteriorate.
  • Unless prescribed by a veterinary doctor, never use on dogs or cats products for livestock that are not explicitly approved for such use. There is a high risk of overdosing or of adverse drug reactions due to ingredients that are not tolerated by pets or are even toxic to them.

Antidote and Treatment of Flumethrin Intoxication

  • There is no antidote for flumethrin poisoning.
  • Treatment consists in preventing further exposure together with supportive and symptomatic measures.
  • In case of dermal exposure rinse the skin with abundant water and soft detergents.
  • After accidental ingestion administer activated charcoal (2g/kg), magnesium sulphate or sodium sulphate (0.5 mg/kg in a 10% aqueous solution)
  • Spasms can be treated with anticonvulsants (e.g. diazepam). If ineffective, fenobarbital or pentobarbital can be tried.
  • Hypersalivation can be treated with atropine.
  • In case of strong vomit and/or diarrhea rehydration measures should be considered.
  • Calcium gluconate and vitamins of the B complex can be used to protect the liver.

Pharmacokinetics of Flumethrin

  • Topically administered flumethrin remains mostly on the hair-coat of the treated animals and is very poorly absorbed through the skin. In contrast with natural pyrethrins and older synthetic pyrethroids flumethrin is quite resistant to UV-light, which allows a residual effect between 5 and 10 days for most sprays and dips.
  • Treated animals can ingest flumethrin through licking or grooming. A large amount of it is excreted unchanged through the feces. The absorbed flumethrin is quickly metabolized in the liver to inactive metabolites that are excreted through urine. This is done by a specific enzyme called glucuronidase. However, cats lack this enzyme and cannot properly metabolize most synthetic pyrethroids. This is why most synthetic pyrethroids are toxic to cats.
  • As a general rule flumethrin products are approved for use on dairy animals in many countries.

Environmental Toxicity of Flumethrin

  • Flumethrin, as all synthetic pyrethroids is extremely toxic to fish and aquatic invertebrates (more in cold than in warm water). For this reason disposal of flumethrin residues (e.g. in empty containers) in watercourses must be absolutely avoided. Disposal of old dip wash charged with flumethrin (or other synthetic pyrethroids) into watercourses is strictly forbidden worldwide because it would have catastrophic consequences for fish and other aquatic animals. There are countries where products for livestock dipping containing synthetic pyrethroids have been withdrawn by the regulatory authorities for this reason.
  • In contrast with organophosphates flumethrin (as most synthetic pyrethroid) is not toxic to birds.
  • There is a certain environmental risk of water pollution from run-off after pour-on administration to large cattle herds. However this risk is substantially lower than the one associated with the use of synthetic pyrethroids in crop pesticides.
  • Correct use on livestock and pets is unlikely to result in any significant environmental pollution.
  • Flumethrin is quite resistant to photodegradation, i.e. exposed to sunlight it breaks down rather slowly.
  • Flumethrin is almost insoluble in water and tends to bind to soil particles. Therefore groundwater contamination is unlikely to occur. Persistence in water depends on pH and temperature.
  • Soil bacteria contribute to the biodegradation of flumethrin.
  • Flumethrin does not bioaccumulate.

Additional information

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

  • Flumethrin belongs to the chemical class of the synthetic pyrethroids and to the so-called type-II pyrethroids (with a cyano group in their molecular structure).
  • Flumethrin used in most products is a mixture of various isomers, normally >90% trans-Z-1 and trans-Z-2 isomers (with <2% cis-Z and <1% trans-E isomers as by-products).
  • Flumethrin is not used in human medicines.
  • Flumethrin is not used in crop pesticides.
  • Flumethrin is not used in public or domestic hygiene as a biocide.
  • 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.
  • Click here for technical and commercial information on flumethrin.


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.