WHO Acute Hazard classification: class U, unlikely to present acute hazard.

Mechanism of action of Phenothrin

Synthetic pyrethroids, including phenothrin, 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 Phenothrin

• LD50 acute, rats, p.o. >5000 mg/kg.
• LD50 acute, rats, dermal, >2000 mg/kg
• As a general rule, dogs tolerate phenothrin 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 and phenothrin is particularly toxic to cats at doses 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 Phenothrin Poisoning

• The primary symptoms of intoxication with phenothrin 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.

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

• Do not administer phenothrin topically (spot-on, shampoos, soaps, sprays, etc.) in case of extended skin lesions: this can lead to an excessive absorption through the damaged skin.
• 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 in cats that are approved only for dogs: synthetic pyrethroids can be toxic to cats.
• Never use spot-ons for large dogs in 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.
• Serious problems with adverse reactions after use of certain flea and tick spot-ons have been reported in the USA, especially on cats and small dogs. According to a report by the EPA from 2010, most problems occurred with spot-ons containing permethrin, phenothrin, cyphenothrin (all are synthetic pyrethroids) and amitraz, not approved for use on cats but erroneously used on them. There have been numerous overdosing cases of small dogs too, apparently because some users buy large vials for big dogs but use them several times in smaller dogs to save money. It seems also that small dogs are more sensitive than large ones and don't tolerate the treatment as large dogs. It also seems that some insufficiently investigated inert ingredients (e.g. solvents) in the formulations are not as harmless as they were supposed to be.
• Several spot-ons (= pipettes, squeeze-ons, drops, etc) for dogs contain high concentrations of phenothrin (>80%!). This results in dermal doses of >100 mg/kg. Mistakes during administration (e.g. overestimating the dog's weight, or using a spot-on vial for larger dogs) can easily double this dose. Comparable pour-on products for cattle containing also synthetic pyrethroids can have an irritant effect on individual animals already at doses <10 mg/kg. It is therefore not surprising that some dogs (e.g. breeds with sensitive skin, old animals, puppies, etc.) do not tolerate such high phenothrin doses.

Antidote and Treatment of Phenothrin Intoxication

• There is no antidote for phenothrin 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 Phenothrin

• Topically administered phenothrin remains mostly on the hair-coat of the treated animals and is very poorly absorbed through the skin.
• Treated animals can ingest phenothrin through licking or grooming. Ingested or absorbed phenothrin is largely metabolized in the liver to non-toxic metabolites. This is done by a specific enzyme called glucuronidase. However, cats lack glucuronidase and cannot properly metabolize most synthetic pyrethroids. This is why most synthetic pyrethroids are toxic to cats. 
• Excretion of metabolites is completed up to 96% 6 days after oral administration, both through urine and feces, depending on the isomer.

Environmental Toxicity of Phenothrin

• Phenothrin, as all synthetic pyrethroids is extremely toxic to fish and aquatic invertebrates. For this reason disposal of phenothrin residues (e.g. in empty containers) in watercourses must be absolutely avoided. 
• In contrast with organophosphates phenothrin (as most synthetic pyrethroid) is not toxic to birds.
• Correct use on dogs is unlikely to result in any significant environmental pollution.
• Phenothrin is quite sensitive to photodegradation, i.e. it breaks down when exposed to sunlight.
• Phenothrin is poorly soluble in water and tends to bind to soil particles. Therefore groundwater contamination is unlikely to occur.
• Soil bacteria contribute to the biodegradation of phenothrin.
• Phenothrin has only low potential for bioaccumulation.

Additional information

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

• Phenothrin belongs to the chemical class of the synthetic pyrethroids and to the so-called type-I pyrethroids (without a cyano group in their molecular structure).
• Phenothrin used in most products is a mixture of isomers with a cis:trans ratio 20:80.
• Phenothrin is not used in livestock.
• Phenothrin is used in human medicines against head lice and other parasites.
  
• Phenothrin is used in crop pesticides.
• Phenothrin is used in public or domestic hygiene as a biocide, e.g. in fumigation products against household pests (ants, cockroaches, termites, etc.)
• 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 phenothrin.