hallucinogen

Meaning: A hallucinogen is a psychoactive drug that changes perception, producing distorted sensations, vivid imagery or hallucinations (perceptions with no external stimulus). Classic examples are LSD, psilocybin (mushrooms) and mescaline (peyote). LSD and psilocybin act mainly on serotonin receptors, especially 5-HT2A. Marijuana (THC) is sometimes grouped here because it can alter perception, though many texts treat it separately. PCP and ketamine are sometimes called dissociative hallucinogens.

On the MCAT: Expect a passage describing a user who sees sounds or colors and a question asking you to classify the drug as a depressant, stimulant, opiate or hallucinogen. You may also link a drug to a neurotransmitter system. Hallucinogens generally cause less physical dependence than opioids or alcohol, but tolerance to LSD can develop quickly.

Common confusion: A hallucination is a percept with no stimulus, while an illusion is a misreading of a real stimulus. Hallucinogens can cause both. Hallucinations also occur without drugs, for example as a positive symptom of schizophrenia, so a hallucination alone does not mean drug use. Classify a drug by its main effect on perception, not by whether the user feels energized.