LSD, PCP, mescaline and peyote are classified as hallucinogens. Hallucinogens interrupt the brain messages that control the intellect and keep instincts in check. Large doses can produce convulsions and coma, heart and lung failure. Chronic users complain of persistent memory problems and speech difficulties for up to a year after their use. Because the drug stops the brain's pain sensors, drug experiences may result in severe self-inflicted injuries. Persistent memory problems and speech difficulties may linger.