Over-the-counter lotions and shampoos that contain pyrethrin or 1% permethrin are often the first choice. The package directions should be followed exactly. These products may continue to kill lice for 2 weeks after treatment; many clinicians recommend a second treatment 7 to 9 days after the first. Side effects of permethrin may include burning or stinging, itching, red skin, or numbness. Prescription-strength 5% permethrin, malathion lotion, or benzyl alcohol lotion may be needed. Lice are becoming resistant to permethrin, so the other medications may be used.