Rapid acting and long lasting pyrethroid insecticide which is a commonly found active ingredient in collars, sprays, shampoos, dips and topical concentrates for control of fleas and other insects. Permethrin is one of the active ingredients in R.I.P. Fleas Extra. It kills adult fleas and flea larvae within minutes to hours of exposure (depending on the dose received). Permethrin is toxic to cats.