Pharmacologic Treatment of Head Lice
Pharmacologic treatment of head lice infestation is focused on three general mechanisms: neurotoxicity resulting in paralysis of the lice (insecticidal treatments), suffocation via "coating" the lice, or dissolution of the wax covering on the exoskeleton (Table 1[1,2,4,8,11–17]). Insecticidal agents that are neurotoxic to lice include permethrin 1% lotion or shampoo (Nix), pyrethrins 0.3%/piperonyl butoxide 4% shampoo (Rid), malathion 0.5% lotion (Ovide), spinosad 0.9% suspension (Natroba), ivermectin 0.5% lotion (Sklice), and oral ivermectin (Stromectol; off-label use). Permethrin 1% is recommended as first-line treatment for head lice.[1,8,18]
Noninsecticidal agents that rely on suffocation or exoskeleton dissolution include benzyl alcohol 5% lotion (Ulesfia), dimethicone solution (Nix Ultra, Lice MD), and isopropyl myristate solution (Resultz; approved by the U.S. Food and Drug Administration in May 2017 but not yet marketed in the United States). The Canadian Paediatric Society recommends dimethicone solution and isopropyl myristate solution as second-line agents if permethrin fails after two treatments.
A key to formulating an effective treatment regimen is recognizing the effectiveness of available treatments in destroying viable eggs because this dictates if retreatment is necessary. Malathion, spinosad, and topical ivermectin are considered ovicidal, and they will kill both live lice and eggs in one treatment. Nonovicidal agents (permethrin, pyrethrins, benzyl alcohol, dimethicone, oral ivermectin, and isopropyl myristate) typically require a repeat application for complete eradication. Timing for nonovicidal treatments is based on the life cycle of the louse. An initial application followed by a second application seven to 10 days later (nine days is optimal) should be sufficient to eradicate most lice. Some authors have postulated that the most effective retreatment schedule for permethrin or pyrethrins might be three doses, on days 0, 7, and 13 to 15.[1,20]
Resistance to permethrin and pyrethrins/piperonyl butoxide can be significant, although the geographic distribution of resistant lice is not well-known. Pseudoresistance may be due to poor adherence, incorrect product use (underdosing or not following directions), and reinfestation. After trials of two appropriately administered courses of permethrin, an alternative agent should be used.
Am Fam Physician. 2019;99(10):635-642. © 2019 American Academy of Family Physicians