Permethrin is a medication and insecticide used to treat scabies and lice. As a medication, it is applied topically as a cream or lotion to kill the parasites. As an insecticide, it can be sprayed on clothing and nets. It works by acting on the nervous system of insects and parasites, causing muscle spasms and paralysis. It is used to treat conditions like scabies, pubic lice, head lice, and crusted scabies. When applied topically, it is poorly absorbed and any amount absorbed is rapidly metabolized and excreted in urine. Common side effects include skin rash and redness. It is generally safe in pregnancy.