The allethrins are a group of related synthetic compounds used in insecticides. They are synthetic pyrethroids, a synthetic form of a chemical found naturally in the chrysanthemum flower. They were first synthesized in the United States by Milton S. Schechter in 1949. Allethrin was the first pyrethroid.
The compounds have low toxicity for humans and birds, and are used in many household insecticides such as RAID as well as mosquito coils. It is highly toxic to fish and aquatic invertebrates. At normal application rates, allethrin is slightly toxic to bees.  Insects subject to exposure become paralyzed (nervous system effect) before dying. Allethrins are toxic to cats because they either do not produce, or produce less of certain isoforms of glucuronosyltransferase, which serve in hepatic detoxifying metabolism pathways.
Allethrin I and allethrin II differ by having a methyl group and a methyl ester, respectively, on one terminus. Each of these allethrins consists of the eight possible stereoisomers. A partly enantiopure variant of allethrin I, consisting of only two stereoisomers in an approximate ratio of 1:1, is called bioallethrin. The same mixture of isomers, but in an approximate ratio of 3:1, is known as esbiothrin.
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- Oregon State University (1996). Allethrin. Retrieved October 26, 2005.
- Illinois Department of Public Health Pyrethroid Insecticides Fact Sheet. Retrieved October 26, 2005.
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- Pyrethrins and Pyrethroids Fact Sheet - National Pesticide Information Center
- Allethrin Pesticide Information Profile - Extension Toxicology Network