Anti-Lice Shampoo — Active Ingredients (Pediculicidal Agents)

🥇 First-Line (Over-the-Counter)

💊 Prescription-Only Agents

How Each Works

• Permethrin & Pyrethrins: Synthetic/natural pyrethroids — disrupt sodium channel function in the louse nerve membrane → paralysis and death. Pyrethrins are derived from chrysanthemum flowers and are combined with piperonyl butoxide (a synergist that inhibits the louse's ability to metabolize the pyrethrin).
• Malathion: Organophosphate cholinesterase inhibitor — far more toxic to insects than humans because human plasma carboxylesterases rapidly hydrolyze it. Both pediculicidal and partially ovicidal.
• Spinosad: Fermentation-derived compound that acts on nicotinic acetylcholine and GABA receptors in insects → paralysis. Ovicidal, so no nit-combing required.
• Abametapir: Metalloprotease inhibitor — disrupts molting enzymes critical for egg development → ovicidal and pediculicidal.
• Ivermectin (topical): Glutamate-gated chloride channel activator → paralysis of lice. Single application; no retreatment needed.
• Lindane: Organochloride that is absorbed through the central nervous system of insects. Significant neurotoxic risk in humans — use only when all other treatments have failed.

How to Use (General Instructions)

• Permethrin 1%: Wash hair with a non-conditioning shampoo, towel-dry, apply to scalp and entire hair length, leave 10 minutes, rinse. Repeat in 9–10 days.
• Pyrethrin shampoo: Apply to dry hair, leave 10 minutes, rinse. Repeat in 9–10 days (no residual activity).
• Malathion 0.5%: Apply to dry hair, leave 8–12 hours to dry naturally, then wash off. Can repeat in 7–9 days if live lice persist.
• Lindane 1%: Apply 30 mL to dry scalp for 4 minutes only, then rinse. Single use only — do not reapply.

Important Notes

• Resistance to permethrin and pyrethrins is increasingly documented in the US — if OTC treatment fails after two applications, move to a prescription agent (malathion or spinosad).
• Pyrethrins are contraindicated in individuals allergic to chrysanthemums or ragweed.
• Two-treatment protocol: Most agents (except ivermectin lotion, spinosad, abametapir) require a repeat application 7–10 days later to kill newly hatched lice (nymphs) that survived the first treatment as eggs.
• Rinse hair over a sink (not in the shower) using warm water to minimize skin absorption.
• Treat all household contacts simultaneously.

• More detail on a specific ingredient (e.g., permethrin, malathion)?
• Which one to choose for a specific situation (child, resistant lice, pregnancy)?
• Side effects of a particular treatment?
• Something completely different?

Anti-Lice Shampoo Active Ingredients

1. 🟢 Permethrin (1%)

• Type: Synthetic pyrethroid
• Form: Lotion / cream rinse
• Mechanism: Blocks sodium channels in louse nerve cells → paralysis → death
• Use: Apply to wet hair for 10 min, rinse. Repeat after 9–10 days
• OTC: Yes
• Safe for: Infants ≥2 months

2. 🟢 Pyrethrins + Piperonyl Butoxide (PBO)

• Type: Natural pyrethroid + synergist
• Pyrethrins: 0.33% | PBO: 4%
• Mechanism: Pyrethrins paralyze lice; PBO blocks the enzyme that lets lice detoxify pyrethrins
• Use: Apply to dry hair for 10 min, rinse. Repeat after 9–10 days
• OTC: Yes (RID, Pronto, A-200)
• ⚠ Avoid in: Chrysanthemum / ragweed allergy

3. 🟡 Malathion (0.5%)

• Type: Organophosphate
• Form: Lotion (Ovide)
• Mechanism: Inhibits cholinesterase in insects; humans inactivate it rapidly via plasma enzymes
• Use: Apply to dry hair, leave 8–12 hours, shampoo off. Repeat in 7–9 days if needed
• Rx: Prescription only
• ⚠ Avoid in: Children <2 years; flammable — keep away from heat/flame

4. 🟡 Spinosad (0.9%)

• Type: Fermentation-derived spinosyn
• Form: Suspension (Natroba)
• Mechanism: Acts on nicotinic acetylcholine + GABA receptors → louse paralysis and death
• Use: Apply to dry hair, leave 10 min, rinse. Single dose usually sufficient
• Rx: Prescription only
• Advantage: Ovicidal — kills eggs too; no nit-combing required

5. 🟡 Ivermectin (0.5% topical)

• Type: Macrocyclic lactone
• Form: Lotion (Sklice)
• Mechanism: Activates glutamate-gated chloride channels → paralysis of lice
• Use: Apply to dry hair, leave 10 min, rinse. Single application only
• OTC: Yes (recently)
• Safe for: ≥6 months

6. 🟡 Abametapir (0.75%)

• Type: Metalloprotease inhibitor (novel)
• Form: Lotion (Xeglyze)
• Mechanism: Inhibits enzymes essential for louse egg development → ovicidal
• Use: Single application
• Rx: Prescription only

7. 🔴 Lindane (1%) — Last Resort

• Type: Organochloride
• Form: Shampoo
• Mechanism: Disrupts CNS of lice
• Use: Apply for 4 minutes only, single use
• ⚠ High risk: Neurotoxic — contraindicated in children, elderly, pregnant women, seizure patients, and those weighing <50 kg
• Use only if all safer treatments have failed

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