PERMETHRIN
Clinical safety rating
safeAnimal studies have demonstrated safety
Permethrin is a type I pyrethroid that acts on the nervous system of parasites by prolonging the inactivation of sodium channels, leading to repetitive neuronal firing and paralysis.
| Metabolism | Permethrin is primarily metabolized by ester hydrolysis to inactive metabolites, with minor contributions from cytochrome P450 enzymes. |
| Excretion | Permethrin is extensively metabolized via ester hydrolysis and oxidation. Metabolites are excreted primarily in the urine (approximately 70-80% of the dose) as glucuronide and sulfate conjugates, with lesser amounts in feces (20-30%). Less than 2% is excreted unchanged. |
| Half-life | The terminal elimination half-life is approximately 12-17 hours in healthy adults. In children and elderly patients, half-life may be prolonged due to reduced esterase activity, with values up to 24 hours. |
| Protein binding | Approximately 90-95% bound to plasma proteins, primarily albumin and alpha-1-acid glycoprotein. |
| Volume of Distribution | Apparent volume of distribution is 2-6 L/kg, indicating extensive tissue distribution, particularly to skin, fat, and brain. High Vd supports prolonged cutaneous retention. |
| Bioavailability | Topical: Systemic absorption is <2% of the applied dose. Oral: Bioavailability is approximately 60-70% due to first-pass metabolism. Permethrin is not administered orally for clinical use. |
| Onset of Action | Topical application: Pruritus relief begins within 12-24 hours; scabies mite death occurs within 10-30 minutes of application. Oral administration (not approved): Onset of CNS effects (if toxic) within 1-4 hours. |
| Duration of Action | Topical: A single application is typically effective for scabies treatment; residual activity on skin persists for up to 14 days, but re-infestation can occur. For head lice, ovicidal activity lasts for 7-10 days. |
| Molecular Weight | 391.29 |
For scabies: Apply 5% cream to entire body from neck to soles of feet, leave on for 8–14 hours, then wash off. For head lice: Apply 1% lotion to damp hair, leave on for 10 minutes, then rinse. Repeat in 7–10 days if necessary.
| Dosage form | LOTION |
| Renal impairment | No dosage adjustment required for any degree of renal impairment. |
| Liver impairment | No dosage adjustment required for any Child-Pugh class. |
| Pediatric use | For scabies in infants and children: 5% cream applied as in adults, including face, scalp, and ears (avoid eyes and mouth). For head lice in children ≥2 months: 1% lotion applied as in adults. |
| Geriatric use | No specific dosage adjustment; use standard adult dosing. Caution in elderly with extensive dermatitis due to increased percutaneous absorption. |
| 1st trimester | Topical permethrin is considered low risk; systemic absorption is minimal (<2%). No increased risk of major malformations reported in human studies. |
| 2nd trimester | Topical permethrin is considered safe; minimal systemic absorption limits fetal exposure. Benefit outweighs risk for scabies/pediculosis treatment. |
| 3rd trimester | Topical permethrin is considered safe; minimal systemic absorption. Use as directed for indicated conditions. |
Clinical note
No significant drug interactions For topical use only if ingestion occurs can cause CNS effects.
| Placental transfer | Permethrin is a pyrethroid insecticide with low systemic bioavailability after topical application. Systemic absorption is <2% in humans, and no studies report quantifiable placental transfer. Animal studies show minimal placental transfer after oral dosing. |
| Breastfeeding | Topical permethrin has negligible systemic absorption, making significant transfer into breast milk unlikely. It is considered compatible with breastfeeding; avoid application to the breast area to prevent infant ingestion. |
| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | Permethrin is a pyrethroid insecticide with low teratogenic potential. In animal studies at doses up to 400 mg/kg/day (maternal toxic doses), no fetal malformations were observed. Human data from topical use during pregnancy (including first trimester) do not indicate increased risk of major congenital anomalies. However, systemic absorption is minimal (<2% with topical application). The FDA assigns pregnancy category B. No specific known fetal risks by trimester. |
| Fetal Monitoring | No specific maternal or fetal monitoring is required beyond standard prenatal care. Monitor for maternal skin irritation or allergic reaction at application site. |
| Fertility Effects | No adverse effects on fertility reported in animal studies at clinically relevant doses. Human data on fertility are absent, but due to low systemic absorption, effects are unlikely. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | lice |
| Serious Effects |
Hypersensitivity to permethrin, any pyrethroid, or any component of the formulation
| Precautions | Seizure risk, especially in children, Hypersensitivity reactions, Avoid contact with eyes and mucous membranes, Use with caution in patients with skin conditions (e.g., atopic dermatitis) |
| Food/Dietary | No known food interactions. Permethrin is applied topically and does not have systemic absorption that would be affected by food. Avoid ingestion; if accidentally ingested, seek medical attention immediately. |
| Clinical Pearls | Permethrin is a synthetic pyrethroid used as a first-line topical treatment for scabies and lice. Apply from neck to toes for scabies, leave on for 8-14 hours; for lice, apply to dry hair and rinse after 10 minutes. Avoid use on open wounds or mucous membranes. Resistance is rare but reported; consider alternative if no response after two treatments. Can cause mild burning or stinging; antihistamines may help pruritus post-treatment. |
| Patient Advice | Apply permethrin to clean, dry skin or hair as directed. · For scabies, cover entire body from neck to soles of feet; avoid eyes, mouth, and nose. · Leave cream on for 8-14 hours (overnight) before washing off. · For lice, apply to dry hair, leave for 10 minutes, then rinse and use a fine-toothed comb. · Do not use more than once a week; two treatments may be needed 7 days apart. · Wash all clothing, bedding, and towels in hot water and dry on high heat. · Avoid sexual contact until treatment is complete and symptoms resolve. · Notify sexual partners and close contacts to seek evaluation and treatment. · Do not share personal items like combs, hats, or clothing. · Itching may persist for up to 2 weeks after successful treatment; do not retreat unless live mites or nits are seen. |
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