The safest and most effective option is to simply comb the lice and nits out of the hair using a specialist long toothed metal nit comb. It takes patience and time, but lice cannot become immune to manual removal.
Alternatively you can use the UK’s unique nit and head lice clearing service – The Hairforce – which lice cannot become immune to, ever. They use a combination of clever technology and forensic hand clearing to clear them. They do all the work for you and guarantee 100% to shut the infestation down.