Allethrins are a group of related synthetic compounds used in insecticides. They are classified as pyrethroids, synthetic versions of pyrethrins, chemicals that are naturally found in chrysanthemums and have insecticidal properties. They were first synthesized in the United States in 1949 by Milton S. Schechter. Allethrins were the first pyrethroids.
They are commonly used in ultra-low volume sprays for outdoor mosquito control, as well as in many household insecticides such as RAID, and mosquito coils.
Allethrin I and Allethrin II differ by having a methyl group and a methyl ester, respectively, at one end. Each of these allethrins consists of eight possible stereoisomers. A partially enantiomerically pure variant of allethrin I, consisting of only two stereoisomers in an approximately 1:1 ratio, is called bioallethrin. The same mixture of isomers, but in an approximate 3:1 ratio, is known as escin.
Long-term exposure to allethrins alters plasma biochemical profiles in humans and may have adverse health effects.  Bioallethrin has been shown to cause oxidative damage, cytotoxicity, and necrosis in human lymphocytes studied in vitro.  It is highly toxic to fish and aquatic invertebrates. At normal application rates, allethrin is slightly toxic to bees.  Exposed insects are paralyzed before death (neurological effects). Allethrins are toxic to cats  because they do not produce or produce less of certain isoforms of glucuronosyltransferases that play a role in hepatic detoxification metabolic pathways.