A hallucinogen is a pharmacological substance that when ingested causes hallucinations - profound distortions in a person's perceptions of reality. Under the influence of hallucinogens, people see images, hear sounds, and feel sensations that seem real but do not exist. Some hallucinogens also produce rapid, intense emotional swings. Hallucinogens cause their effects by disrupting the interaction of nerve cells and the neurotransmitter serotonin. Hallucinogenic substances may be synthetically manufactured in a laboratory, like LSD, or derived from natural plant (e.g., peyote, cannabis), fungus (e.g., psilocybe mushrooms) or animal (e.g., Colorado River toad) sources.
Only a few hallucinogens produce actual hallucinations, where things that are not real appear to be realistic.
In the United States, hallucinogens are classified as Schedule I drugs.