1 After each treatment, checking the hair and combing with a nit comb to remove nits and lice every 2–3 days may decrease the chance of self–reinfestation. 2  Continue to check for 2–3 weeks to be sure all lice and nits are gone. The medicine may take longer to kill all the lice. 2  Comb dead and any remaining live lice out of the hair using a fine–toothed nit comb. 3  If, after 8–12 hours of treatment, no dead lice are found and lice seem as active as before, the medicine may not be working.