Piperonyl butoxide (PBO) is an organic compound used as a component of pesticide formulations. It is a waxy white solid. It is a semisynthetic derivative of safrole. It is used for the treatment of head, pubic (crab), and body lice. Piperonyl butoxide is a synergist. It has no pesticidal activity of its own, but acts to increase the activity of pesticides such as carbamates, pyrethrins, pyrethroids, and rotenone. PBO acts as an insecticide synergist by inhibiting the natural defense mechanisms of the insect, the most important of which is the Mixed Function Oxidase system, also known as the cytochrome P-450 system. The MFO system is the primary route of detoxification in insects, and causes the oxidative breakdown of insecticides like pyrethrins and the synthetic pyrethroids - thus when PBO is added, higher insecticide levels remain in the insect to exercise their lethal effect. An important consequence of this property is that, by enhancing the activity of a given insecticide, less may be used to achieve the same result. PBO does not appear to have a significant effect on the MFO system in humans. Skin Sensitization PBO has a low acute toxicity by oral, inhalation and dermal routes. It is minimally irritating to the eyes and skin. It is a not a dermal sensitizer. No evidence suggests that PBO disrupts the normal functioning of the endocrine system. This includes the recently developed data to assess the possible interaction of PBO with the endocrine system.