Classic psychedelics are a group of psychoactive drugs that includes lysergic acid diethylamide (LSD), psilocybin, N,N-dimethyltryptamine (DMT) and mescaline. Despite having distinct characteristics, these compounds are often grouped together as they all act as agonists of serotonin 5-hydroxytryptamine (5-HT) 2A receptors, which is how they induce psychoactive effects, and because they are found in nature, e.g. LSD is derived from naturally occurring ergotamine.