The only 100% effective method of getting rid of headlice is manual removal. This can be done with products to make it easier, but can also be done with just conditioner and a good quality nit comb. You need to remove all live lice (beware they like to run away and hide) and nits (eggs) that are stuck to the hair shaft. Eggs can be stuck on any side of the hair shaft so each hair must be combed from every angle with the comb.
Resistance
Lots of lice can now survive chemical treatments.