Elimax® has a physical mode of action and does not contain pesticides, which means that head lice cannot develop resistance to its ingredients. Reasons for unsuccessful treatment can be: the wrong diagnosis, not following the instructions (not using enough product, stopping treatment too soon, not removing enough lice or nits with the comb, etc.), (re-)infestation by untreated classmates or family members, not checking and repeating the treatment if necessary 7 days after initial treatment.