Permethrin

# Permethrin ## Overview - **Classification**: Pyrethroid insecticide; Topical scabicide and pediculicide. - **Mechanism**: Acts as a neurotoxin, disrupting neuron membrane function by binding to sodium channels, leading to prolonged depolarization, paralysis, and death of parasites. ## Primary Indications 1. **Scabies**: Treatment of infestation with *Sarcoptes scabiei*. 2. **Head Lice**: Treatment of infestation with *Pediculus humanus capitis*. ## Adult Dosing ### Standard Dosing **Scabies (5% Permethrin Cream)** - **Dose**: Apply a thin layer of **5% permethrin cream** to the entire body from the neck down. - **Frequency**: Single application. - **Route**: Topical. - **Duration**: Leave on for **8-14 hours**, then wash off thoroughly. - **Special Considerations**: A re-treatment is typically recommended in **7-14 days** if live mites are seen or new lesions appear. **Head Lice (1% Permethrin Lotion/Cream Rinse)** - **Dose**: Apply **1% permethrin lotion/cream rinse** to washed, towel-dried hair and scalp. - **Frequency**: Single application. - **Route**: Topical to hair/scalp. - **Duration**: Leave on for **10 minutes**, then rinse thoroughly. - **Special Considerations**: Re-treat in **7-10 days** if live lice are seen. ### Dose Adjustments - **Renal Impairment**: Not required. Minimal systemic absorption. - **Hepatic Impairment**: Not required. Minimal systemic absorption. - **Elderly Patients**: No specific adjustments needed. Follow adult dosing. ## Pediatric Dosing ### Neonates (0-28 days) - **Scabies/Lice**: **Not recommended** as first-line. - **Special Notes**: Safety and efficacy not established. Use only after consulting a specialist, and if clearly indicated, due to potential for increased systemic absorption and immature metabolism in very young infants. ### Infants (2 months - 12 months) - **Scabies (5% Permethrin Cream)** - **Dose**: Apply **5% permethrin cream** to entire body, including scalp, face, and ears (avoiding eyes/mouth). - **Frequency**: Single application. - **Duration**: Leave on for **8-14 hours**, then wash off. - **Special Notes**: Re-treat in **7-14 days** if live mites/new lesions. - **Head Lice (1% Permethrin Lotion/Cream Rinse)** - **Dose**: Apply **1% permethrin lotion/cream rinse** to washed, towel-dried hair and scalp. - **Frequency**: Single application. - **Duration**: Leave on for **10 minutes**, then rinse. - **Special Notes**: Re-treat in **7-10 days** if live lice seen. ### Children (1-12 years) - **Scabies (5% Permethrin Cream)** - **Dose**: Apply **5% permethrin cream** to entire body, including scalp, face, and ears (avoiding eyes/mouth). - **Frequency**: Single application. - **Duration**: Leave on for **8-14 hours**, then wash off. - **Special Notes**: Re-treat in **7-14 days** if live mites/new lesions. - **Head Lice (1% Permethrin Lotion/Cream Rinse)** - **Dose**: Apply **1% permethrin lotion/cream rinse** to washed, towel-dried hair and scalp. - **Frequency**: Single application. - **Duration**: Leave on for **10 minutes**, then rinse. - **Special Notes**: Re-treat in **7-10 days** if live lice seen. ### Adolescents (13-18 years) - **Dose**: Follow adult dosing guidelines for both indications. ## Safety Information ### Contraindications - **Absolute**: Known hypersensitivity to permethrin, other pyrethroids, or pyrethrins. ### Common Adverse Effects - **Common (1-10%)**: Pruritus (itching), erythema (redness), stinging, burning, tingling, edema (swelling), rash, numbness. - **Serious but Rare**: Severe allergic reactions (e.g., anaphylaxis). ### Key Drug Interactions - No significant systemic drug interactions are reported due to minimal topical absorption. ## Monitoring & Follow-up - **Before Treatment**: Confirm diagnosis (visual inspection, skin scraping for scabies). - **During Treatment**: Monitor for skin irritation or allergic reactions. - **Clinical Signs**: For scabies, persistence of live mites or new burrows after re-treatment. For lice, persistence of live lice after re-treatment. - **Important**: Post-scabies itch can persist for several weeks after successful treatment due to residual irritation from dead mites. ## Clinical Pearls - 💡 **Scabies**: Treat all close contacts simultaneously, even if asymptomatic. Wash all clothing, bedding, and towels in hot water or seal in a plastic bag for 72 hours. - 💡 **Head Lice**: Nit combing is crucial after treatment. Do not use conditioner before applying lice treatment, as it can reduce efficacy. - 💡 **Application**: Avoid contact with eyes, mouth, and mucous membranes. If contact occurs, rinse thoroughly with water. - 💡 **Retreatment**: The second application (for both indications) is critical to kill newly hatched parasites. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.