The general group of pharmacological agents commonly known as hallucinogens can be divided into three broad categories: psychedelics, dissociatives, and deliriants. These classes of psychoactive drugs have in common that they can cause subjective changes in perception, thought, emotion and consciousness. Unlike other psychoactive drugs, such as stimulants and opioids, the hallucinogens do not merely amplify familiar states of mind, but rather induce experiences that are qualitatively different from those of ordinary consciousness. These experiences are often compared to non-ordinary forms of consciousness such as trance, meditation, conversion experiences, and dreams.
One thing that most of these drugs do not do, despite the ingrained usage of the term hallucinogen, is to cause hallucination. Hallucinations, strictly speaking, are perceptions that have no basis in reality, but that appear entirely realistic. A typical “hallucination” induced by a psychedelic drug is more accurately described as a modification of regular perception, and the subject is usually quite aware of the illusory and personal nature of their perceptions. Deliriants, such as diphenhydramine and atropine, may cause hallucinations in the proper sense.
Psychedelics, dissociatives, and deliriants have a long history of use within medicinal and religious traditions around the world. They are used in shamanic forms of ritual healing and divination, in initiation rites, and in the religious rituals of syncretistic movements such as União do Vegetal, Santo Daime, and the Native American Church. When used in religious practice, psychedelic drugs, as well as other substances like tobacco, are referred to as entheogens.
An entheogen, in the strictest sense, is a psychoactive substance used in a religious or shamanic context. Entheogens generally come from plant sources which contain molecules closely related to endogenous neurochemicals. They occur in a wide variety of psychedelics of various religious rites and have been shown to directly provoke what users perceive as spiritual/mystical experiences.