NYDA pump spray contains dimeticone, which is a silicone-based substance.
NYDA works in a different way to conventional insecticides for head lice. Instead of poisoning the parasites by chemical means, the dimeticone kills lice and their eggs by physically coating and suffocating them. Head lice cannot become resistant to NYDA because it works in this physical way.
The head lice are killed within a minute of application but the eggs can take up to eight hours to suffocate. One application should kill both lice and eggs, however, it's a good idea to check for head lice again seven days after treatment. By this time any eggs that survived the treatment will have hatched. If needed it's fine to repeat the treatment straight away.
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