The best product to use to kill head lice is Nix Cream Rinse, available over-the-counter.
If a prescription medication, such as Kwell Shampoo (Lindane) is desired, then the patient should first be examined by a physician due to the potential for seizures and even death from incorrect use of this product.
Most important for a permanent cure is tedious daily nit removal with fingernails or a fine-toothed nit comb for about 2 weeks.
To prevent reinfestation, carefully clean and/or vacuum everything the hair may have touched within the previous few days.