Permethrin is a synthetic pyrethroid insecticide and antiparasitic agent developed in the 1970s and introduced for medical use in the 1980s. It is widely used for the treatment of infestations such as scabies and head lice due to its high efficacy and relatively low toxicity in humans. Its history is marked by its effectiveness in eliminating ectoparasites, along with its favorable safety profile when used topically. Permethrin works by disrupting sodium channel function in the nerve cell membranes of parasites, leading to delayed repolarization, paralysis, and death of the organism. It has become a standard first-line therapy for scabies and lice infestations and is also used in public health programs for parasite control.
BRAND NAMES
Nix (commonly used for head lice treatment)
Elimite (used for scabies treatment)
Acticin
Permite
MECHANISM OF ACTION
Permethrin is a synthetic pyrethroid that acts on the voltage-gated sodium channels in the nerve cell membranes of parasites such as lice and mites. It delays the repolarization of these channels, causing prolonged sodium influx and repetitive nerve impulses. This leads to paralysis and eventual death of the parasite.
PHARMACODYNAMICS
Permethrin is a synthetic pyrethroid that exerts its effect by acting on voltage-gated sodium channels in the nerve cell membranes of parasites such as mites and lice. It prolongs sodium channel activation, leading to delayed repolarization, repetitive nerve firing, paralysis, and eventual death of the parasite. It is highly effective against Sarcoptes scabiei (scabies mites) and Pediculus humanus capitis (head lice), and also has some residual activity that helps prevent reinfestation.
ADMINISTRATION
Permethrin is administered topically as a cream, lotion, or shampoo. For scabies, a 5% cream is applied to the entire body from neck down (and in infants, including the scalp) and left on for 8–14 hours before washing off. For head lice, a 1% lotion or shampoo is applied to the scalp and hair, left for about 10 minutes, and then rinsed. Treatment may be repeated after 7–10 days if needed.
DOSAGE AND STRENGTH
Scabies: Permethrin 5% cream, single application (repeat if necessary after 1 week)
Head lice: Permethrin 1% lotion or shampoo, applied once and repeated if required
Available in topical formulations such as 1% (lice) and 5% (scabies)
DRUG INTERACTIONS
Permethrin has minimal systemic absorption, so clinically significant drug interactions are rare. However, concurrent use with other topical irritants or insecticides may increase the risk of skin irritation or dermatitis.
CONTRAINDICATIONS
Permethrin is contraindicated in patients with known hypersensitivity to permethrin, pyrethroids, or pyrethrins, as exposure may lead to allergic skin reactions or irritation. It should be used with caution in infants younger than two months of age and only under medical supervision when clearly necessary.
SIDE EFFECTS
Mild skin irritation
Burning or stinging sensation
Itching (pruritus)
Redness (erythema)
Rash or allergic contact dermatitis
Tingling or numbness at application site (rare)
Scalp irritation (when used for head lice)
Temporary worsening of itching (especially in scabies, due to mite death
OVER DOSAGE
Permethrin overdose is uncommon because it is applied topically and has very low systemic absorption. However, excessive or inappropriate use may cause skin irritation, redness, burning, itching, and contact dermatitis. If large amounts are applied or ingested accidentally, more severe effects such as nausea, vomiting, headache, dizziness, and, rarely, nervous system symptoms like tremors or seizures may occur, especially in children.
TOXICITY
Permethrin has low systemic toxicity in humans when used topically as directed because it is poorly absorbed through intact skin and rapidly metabolized. However, toxicity may occur with excessive application, ingestion, or prolonged exposure. Local toxic effects include severe skin irritation, burning, redness, itching, and allergic contact dermatitis.
