Piperonyl butoxide (PBO) is a pesticide active ingredient that acts as a synergist, enhancing the pesticidal properties of other chemicals. It is registered for use in combination with insecticide and acaricide active ingredients, such as pyrethrins, synthetic pyrethroids and organophosphates, in a variety of agricultural and non-agricultural sites. Registered residential uses include insect control inside and outside of the home, as well as direct applications to pets. Formulations and application methods are diverse. PBO was first registered in the United States in the 1950s. The Reregistration Eligibility Decision (RED) for PBO was issued in 2006.