Phencyclidine (a shortened version of the chemical name "phenylcyclohexylpiperidine"), usually called PCP, is an illegal drug. PCP is called a hallucinogenic drug: it causes people to hallucinate, and see, hear, and sense things that are not real. PCP is neurotoxic. It causes changes to the brain that can leave permanent damage. Before people started using PCP as an illegal drug, it was used as an anesthetic.