Classical hallucinogens can be divided into 2 chemical classes. Tryptamines include psilocybin (present in several mushroom genera) and the semisynthetic D-lysergic acid diethylamide (LSD). Phenethylamines include mescaline (the active constituent of peyote). Classical hallucinogen effects are primarily mediated by agonist activity at 5-HT2A receptors. Other classes of pharmacologically unrelated substances have sometimes been labeled as “hallucinogens,” including cannabis, 3,4-methylenedioxymetham-phetamine, dissociative anesthetics (e.g., phencyclidine), and anticholinergic drugs (e.g., scopolamine). In this entry, the term hallucinogen will refer exclusively to classical hallucinogens.