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A psychedelic substance is a psychoactive drug whose primary action is to alter cognition and perception. Psychedelics are part of a wider class of psychoactive drugs known as hallucinogens, a class that also includes structurally unrelated substances such as dissociatives and deliriants. Unlike other drugs such as stimulants and opioids which induce familiar states of consciousness, psychedelics tend to affect and explore the mind in ways that result in the experience being qualitatively different from those of ordinary consciousness. The psychedelic experience is often compared to non-ordinary forms of consciousness such as trance, meditation, yoga, religious ecstasy, dreaming and even near-death experiences. With a few exceptions, most psychedelic drugs fall into one of the three following families of chemical compounds; tryptamines [more specifically: alkylated tryptamines], phenethylamines [more specifically: alkoxylated phenethylamines], and lysergamides.
The term psychedelic is derived from the Greek words ψυχή (psyche, "soul, mind") and δηλείν (delein, "to manifest"), hence "soul-manifesting", the implication being that psychedelics can access the soul and develop unused potentials of the human mind. The word was coined in 1957 by British psychiatrist, Humphrey Osmond, the spelling loathed by American ethnobotanist, Richard Schultes, but championed by the American psychologist, Timothy Leary.
Aldous Huxley had suggested to Humphrey Osmond in 1957 his own coinage phanerothymic (Greek "phanero-" visible + Greek "thymic" spiritual, thus "visible spirituality"). Recently, the term entheogenic has come into use to denote the use of psychedelic drugs in a non-recreational context.
An attempt (in no way capable of exhaustively covering the "tremendous range" of psychedelic experience) to give "a [...] general view of their [psychedelics] effects on perception, thought, and feeling" follows:
Psychedelics have a long history of traditional use in medicine and religion, where they are prized for their perceived ability to promote physical and mental healing. In this context, they are often known as entheogens. Native American practitioners using mescaline-containing cacti (most notably peyote, San Pedro, and Peruvian torch) have reported success against alcoholism, and Mazatec practitioners routinely use psilocybin mushrooms for divination and healing. Ayahuasca, which contains the powerful psychedelic DMT, is used in Peru and other parts of South America for spiritual and physical healing as well as in religious festivals.
Classical or serotonergic psychedelics (agonists for the 5-HT2A serotonin receptors) include LSD (also known as "acid"), psilocin (the active constituent of psilocybin mushrooms, commonly known as "magic mushrooms" or "shrooms"), mescaline (the active constituent of peyote), and DMT (the active constituent of ayahuasca and potentially an endogenous psychedelic compound). Salvia divinorum is an atypical psychedelic that has been gaining popularity over the past decade, due to its legality in many US states. It is often compared to DMT due to its short and very intense trip. A few newer synthetics such as MDMA and 2C-B have also enjoyed some popularity. Cannabis is one of the most widely used psychoactive drugs in the world.
This class of psychedelics includes the classical hallucinogens, including the lysergamides like LSD and LSA, tryptamines like psilocybin and DMT, and phenethylamines like mescaline and 2C-B. Many of these psychedelics cause remarkably similar effects, despite their different chemical structure. However, many users report that the three families have subjectively different qualities in the "feel" of the experience, which are difficult to describe. At lower doses, these include sensory alterations, such as the warping of surfaces, shape suggestibility, and color variations. Users often report intense colors that they have not previously experienced, and repetitive geometric shapes are common. Higher doses often cause intense and fundamental alterations of sensory perception, such as synesthesia or the experience of additional spatial or temporal dimensions. Some compounds, such as 2C-B, have extremely tight "dose curves", meaning the difference between a non-event and an overwhelming disconnection from reality can be very slight. There can be very substantial differences between the drugs, however. For instance, 5-MeO-DMT rarely produces the visual effects typical of other psychedelics and ibogaine (a 'complex tryptamine') is also an NMDA receptor antagonist and κ-opioid receptor agonist in addition to being an agonist for the 5-HT2A receptors, resulting in dissociative effects as well (see dissociatives below).
The empathogen-entactogens are phenethylamines of the MDxx class such as MDMA, MDEA, and MDA. Their effects are characterized by feelings of openness, euphoria, empathy, love, heightened self-awareness, and by mild audio and visual distortions (an overall enhancement of sensory experience is often reported). Their adoption by the rave subculture is probably due to the enhancement of the overall social and musical experience. MDA is atypical to this experience, often causing hallucinations and psychedelic effects in equal profundity to the chemicals in the 5-HT2A agonist category, but with substantially less mental involvement, and is possibly both a serotonin releaser and 5-HT2A receptor agonist.
The cannabinoid tetrahydrocannabinol (THC) and related compounds are capable of activating the brain's endocannabinoid system. Some effects may include a general change in consciousness, mild euphoria, feelings of general well-being, relaxation or stress reduction, enhanced recollection of episodic memory, hunger, increased sensuality, increased awareness of sensation, creative or philosophical thinking, disruption of linear memory, paranoia, agitation, anxiety, potentiation of other psychedelics, and increased awareness of sound, patterns, and color.
Certain dissociative drugs acting via NMDA antagonism are known to produce what some might consider psychedelic effects. The main differences between dissociative psychedelics and serotonergic hallucinogens are that the dissociatives cause more intense derealization and depersonalization. For example, ketamine produces sensations of being disconnected from one's body and that the surrounding environment is unreal, as well as perceptual alterations seen with other psychedelics.
Salvia divinorum is a dissociative that is sometimes classified as an atypical psychedelic. The active molecule in the plant, salvinorin A, is a kappa opioid receptor agonist, working on a part of the brain that deals with pain. Activation of this receptor is also linked to the dysphoria sometimes experienced by users of opiates either therapeutically or recreationally. An unusual feature of S. divinorum is its high potency (dosage is in the microgram range) and extremely disorienting effects, which often include "entity contact", complete loss of reality-perception and user's experiencing their consciousness as being housed in different objects e.g. a pane of glass or a pencil. It is also unusual as it is a terpenoid like THC as opposed to an alkaloid like the comparably intense serotonergic psychedelics and NMDA receptor antagonists mentioned above.
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