Hallucinogens are drugs that cause profound distortions of reality through hallucinations and delusions. Some common hallucinogens are LSD, PCP, mescaline, and peyote. LSD is a clear, odorless substance that is one of the most potent hallucinogens. PCP was originally developed as an anesthetic but was discontinued due to serious side effects. Mescaline is found in peyote cacti and has been used in religious ceremonies by some cultures. Hallucinogens do not typically cause physical addiction but can impair cognitive abilities.