Products Not Recommended due to Reports of Potential Side Effects. Pyrethrins are naturally occurring compounds extracted from chrysanthemum plants. Insects possess mechanisms to break down pyrethrins so a synergist, piperonyl butoxide, is usually added to the formula which inhibits the insect s ability to break down the pyrethrins. Permethrin has the same basic chemical make-up but is synthetically enhanced to be more effective. Permethrin has much longer residual activity and does not require the use of a synergist.
Head lice are specific to humans.
They can only survive on a human host. If a head louse is removed from its host, it will die within a day or two. An adult head louse is approximately 2.5 to 3.5 mm long. Experts are currently researching this issue and are trying to determine to what extent resistance exists. The best way to avoid this situation is to follow the directions on the product label closely.