Permethrin is a common synthetic chemical, widely used as an insecticide, acaricide, and insect repellent. It belongs to the family of synthetic chemicals called pyrethroids and functions as a neurotoxin, affecting neuron membranes by prolonging sodium channel activation. It is not known to rapidly harm most mammals or birds, but is dangerously toxic to fish and to cats: in cats it may induce hyperexcitability, tremors, seizures, and even death. In general, it has a low mammalian toxicity and is poorly absorbed by skin. In medicine, permethrin is a first-line treatment for scabies; a 5% (w/w) cream is marketed by Johnson & Johnson under the name Lyclear. In Nordic countries and North America, it is marketed under trade name Nix, often available over the counter. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.