Products
Permethrin is contained in numerous veterinary medicines, plant protection products, in agents against vermin such as insect sprays against wasps, ants, woodworms, moths, and in repellents. In many countries, only one drug was registered with Swissmedic for a long time, namely Loxazol lotion (1%) against head lice. The cream with 5% permethrin against scabies had to be imported from Germany (Infectoscab 5% cream) or manufactured in a pharmacy. In 2018, Scabi-med 5% cream was approved (Permamed AG); see Permethrin cream.
Structure and properties
Permethrin (C21H20Cl2O3, Mr = 391.3 g/mol) belongs to the pyrethroids. These are synthetically produced derivatives of pyrethrins naturally occurring in certain composite flowers. Permethrin exists as a yellow to slightly orange-brown, lipophilic, viscous liquid or as a hardening mass and is poorly soluble in water. Two carbon atoms on the cyclopropane ring are chrial. Permethrin is a mixture of 4 stereoisomers, each consisting of two and two isomers. The -isomers are considered slightly more toxic. Drugs typically contain 25% of the -isomers and 75% of the -isomers.
Effects
Permethrin (ATC P03AC04) has insecticidal, acaricidal (miticidal), and insect repellent properties. It is effective as a contact and frass poison against numerous pests and parasites, including lice, fleas, ticks, mites and mosquitoes. It is a neurotoxin and causes sensory hyperexcitability, incoordination, and fatigue by binding to voltage-gated sodium channels of nerve cells. Permethrin is selective due to the different structure of the sodium channels. However, it is toxic to fish and, at high doses, to cats. Increasing resistance is a problem, due in part to point mutations in the sodium channel. Permethrin is poorly absorbed through the skin, rapidly metabolized by ester hydrolysis, conjugated, and renally excreted.
Indications
Indications for use include:
- For the prevention and treatment of infestation with head lice.
- For the treatment of scabies
- Body or crabs
Permethrin is also used as an insect repellent, to impregnate clothes and mosquito nets, as an insecticide in insect sprays, as a pesticide and as a veterinary medicine for example against ticks and fleas. Some products are not suitable for cats (see below).
Application
According to the package insert.
Contraindications
Permethrin should not be used in cases of hypersensitivity to permethrin, pyrethroids, or pyrethrins. Refer to the drug label for complete precautions. Permethrin should not come into contact with the eyes or mucous membranes and should not be applied near the eyes or to open wounds. It should not get into water because it is harmful to insects and aquatic life such as fish and algae. It is also harmful to bees. Cats are more susceptible and severe to fatal poisonings regularly occur when they are treated with spot-on products against fleas and ticks that are actually intended for dogs. Even small amounts of such products can be fatal to cats!
Interactions
No information is available on possible drug interactions.
Adverse effects
Possible adverse effects include skin afflictions such as itching, burning, stinging, redness, oedema, and allergic reactions. Side effects are sometimes difficult to distinguish from symptoms of the disease. Systemic reactions such as headache or abdominal pain are considered very rare. The drugs are ineffective against resistant insects. Permethrin is not neurotoxic when applied as a drug, like lindane (out of commerce).
