Hallucinogens are drugs that cause hallucinations by disrupting the interaction of nerve cells and the neurotransmitter serotonin in the brain. Under their influence, people see images and hear sounds that do not exist in reality. Some hallucinogens also produce rapid and intense emotional swings. Common hallucinogens include mushrooms, LSD, and PCP. They alter a person's perception of reality and may cause hallucinations or other sensory distortions. Repeated use leads to cross-tolerance, meaning a higher tolerance develops with more frequent use over shorter periods of time.