Usage
- Permethrin is prescribed for the treatment of scabies (infestation with Sarcoptes scabiei mites) and head lice (infestation with Pediculus humanus capitis). It is also used to prevent lice infestation in individuals who have had close contact with someone infested with head lice. For scabies, permethrin is indicated for adults and children older than 2 months. For head lice, it is indicated for adults and children older than 2 months.
- Pharmacological classification: Ectoparasiticide, Antiparasitic agent, Pyrethrin.
- Mechanism of Action: Permethrin acts on the nerve cell membrane of the parasite, disrupting the sodium channel current, ultimately leading to paralysis and death of the mite or louse.
Alternate Names
- No commonly used alternate names.
- Brand Names: Nix, Elimite, Acticin.
How It Works
- Pharmacodynamics: Permethrin interferes with neuronal sodium channel function in arthropods (mites and lice), leading to delayed repolarization, paralysis, and death. It affects the parasite’s nervous system, but due to limited systemic absorption, it has minimal effect on the human nervous system.
- Pharmacokinetics: Topical permethrin is poorly absorbed through intact human skin. The small amount absorbed is rapidly metabolized in the liver by hydrolysis and other metabolic pathways to inactive metabolites, primarily excreted in the urine.
- Mode of Action: Binds to voltage-gated sodium channels in the parasite’s nerve cell membranes, causing prolonged opening of the channels and repetitive nerve impulses, resulting in paralysis and death.
- Receptor binding, enzyme inhibition, or neurotransmitter modulation: Primarily targets voltage-gated sodium channels in the parasite’s nerve cell membranes.
- Elimination Pathways: The small amount absorbed systemically is metabolized in the liver and excreted mainly through the urine.
Dosage
Standard Dosage
Adults:
- Scabies: Apply a thin layer of 5% permethrin cream to the entire body from the neck to the soles of the feet, including under fingernails and toenails, and in skin folds. Leave on for 8-14 hours, then wash off thoroughly. One application is usually sufficient; repeat after 7 days if live mites are still present.
- Head Lice: Apply 1% permethrin lotion or cream rinse to clean, towel-dried hair and scalp, saturating the hair. Leave on for 10 minutes, then rinse thoroughly. Remove nits with a fine-toothed comb. Repeat treatment after 7 days if live lice or nits are still present.
Children:
- Scabies (2 months and older): Apply 5% permethrin cream as directed for adults. For children under 2 years old, apply to the head and neck, including hairline, temples, and forehead. Ensure thorough application to all affected areas.
- Head Lice (2 months and older): Apply 1% permethrin lotion or cream rinse as directed for adults.
Special Cases:
- Elderly Patients: For scabies, apply 5% cream to the entire body, including the head, face, and ears, as mites may infest these areas in older individuals.
- Patients with Renal Impairment: No dose adjustment is typically necessary as minimal systemic absorption occurs.
- Patients with Hepatic Dysfunction: No dose adjustment is typically necessary.
- Patients with Comorbid Conditions: No specific dose adjustments are indicated, but exercise caution in patients with pre-existing skin conditions such as eczema or severe dermatitis.
Clinical Use Cases
Permethrin is not indicated for use in clinical scenarios such as intubation, surgical procedures, mechanical ventilation, ICU use, or emergency situations.
Dosage Adjustments
No specific dosage adjustments are indicated other than as described above for special populations.
Side Effects
Common Side Effects
- Itching, burning, stinging, redness, numbness, tingling, and rash at the application site. Worsening of scabies symptoms can occur temporarily after treatment.
Rare but Serious Side Effects
- Allergic reactions (difficulty breathing, hives, swelling of the face, lips, or tongue). Seizures (rare and association not confirmed).
Long-Term Effects
No significant long-term side effects are known with topical permethrin use.
Adverse Drug Reactions (ADR)
Severe allergic reactions, including anaphylaxis and breathing difficulties, are rare but require immediate medical attention.
Contraindications
- Hypersensitivity to permethrin, pyrethrins, pyrethroids, or any component of the formulation.
- Infants younger than 2 months of age (for 5% cream).
Drug Interactions
No clinically significant drug interactions are known with topical permethrin. Concomitant use of corticosteroids for eczema may exacerbate scabies infestation by reducing the immune response.
Pregnancy and Breastfeeding
- Pregnancy Safety Category: B. Studies in animals have not shown evidence of fetal harm. There are no adequate and well-controlled studies in pregnant women, but the amount of permethrin absorbed systemically after topical application is minimal. Use with caution during pregnancy if potential benefit outweighs risk.
- Breastfeeding: Minimal amounts of permethrin are excreted in breast milk after topical application. Consider temporarily withholding breastfeeding or applying permethrin after nursing and then washing off thoroughly before the next feeding.
Drug Profile Summary
- Mechanism of Action: Disrupts sodium channels in the parasite’s nerve membranes, causing paralysis and death.
- Side Effects: Itching, burning, stinging, redness at the application site. Rarely allergic reactions.
- Contraindications: Hypersensitivity to permethrin or other pyrethrins/pyrethroids. Infants < 2 months.
- Drug Interactions: No significant interactions known. Concomitant use of topical corticosteroids may exacerbate scabies.
- Pregnancy & Breastfeeding: Generally considered safe but use with caution. Minimal excretion in breast milk.
- Dosage: Scabies: 5% cream applied topically for 8-14 hrs. Lice: 1% lotion/rinse applied for 10 mins.
- Monitoring Parameters: Observe for resolution of infestation, signs of skin irritation, and allergic reactions.
Popular Combinations
No specific drug combinations are routinely used with permethrin.
Precautions
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General Precautions: For external use only. Avoid contact with eyes and mucous membranes. Wash thoroughly if contact occurs. Perform pre-screening for known allergies to permethrin or related compounds.
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Specific Populations: Use with caution during pregnancy and breastfeeding.
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Lifestyle Considerations: No specific lifestyle considerations are known to interact with topical permethrin.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for permethrin?
A: Scabies: Adults and children 2 months and older: Apply a thin layer of 5% cream to the entire body from the neck to the soles of the feet, leave on for 8-14 hours, then wash off. Lice: Adults and children 2 months and older: Apply 1% lotion/cream rinse to saturate clean, towel-dried hair, leave on for 10 minutes, then rinse.
Q2: How does permethrin work against scabies and lice?
A: Permethrin acts on the nerve cell membranes of mites and lice, disrupting sodium channel currents and causing paralysis and death.
Q3: Are there any serious side effects I should be aware of?
A: While rare, allergic reactions can occur, requiring immediate medical attention. Seizures have been reported rarely, although a direct link to permethrin use has not been established.
Q4: Can permethrin be used during pregnancy or breastfeeding?
A: Permethrin is generally considered safe for use during pregnancy and breastfeeding, but should be used cautiously. The minimal systemic absorption limits potential fetal or infant exposure. Discuss the risks and benefits with your patient.
Q5: How long does it take for permethrin to work?
A: One application is usually sufficient to eradicate scabies or lice, although a second treatment may be necessary after 7 days for scabies and 7-10 days for lice if live parasites or nits are still present.
Q6: What should I do if permethrin gets in my eyes?
A: Immediately flush the eyes with plenty of water.
Q7: Can I use other topical treatments with permethrin?
A: Avoid using topical corticosteroids for eczema before or during permethrin treatment, as this may worsen scabies.
Q8: What if the itching persists after scabies treatment?
A: Itching may persist for several weeks after scabies treatment due to a hypersensitivity reaction to the dead mites. This does not indicate treatment failure. Advise patients to avoid re-treatment unless live mites are still present after 4 weeks.
Q9: How can I minimize the risk of re-infestation with lice?
A: Advise patients to wash all bedding, clothing, and other items that have come into contact with the infested individual in hot water and dry on high heat. Vacuum furniture and floors. Avoid sharing combs, brushes, hats, and other personal items.
Q10: When should I refer a patient to a dermatologist?
A: Consider referral if the infestation does not respond to treatment, if secondary skin infections develop, or if there are significant skin reactions to permethrin.
