Phenobarbital sedative drug, molecular model. Phenobarbital (phenobarbitone) is a type of barbiturate, a class of drugs that act as sedatives on the central nervous system and brain. It is anticonvulsant and used to reduce the number of seizures in people with epilepsy. The seizures are caused by uncontrolled and excessive electrical brain activity. Phenobarbital's chemical formula is C12H12N2O3. Atoms are represented as spheres and are colour-coded: carbon (C, grey), hydrogen (H, turquoise), nitrogen (N, blue) and oxygen (O, red).