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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 unrelated substances such as dissociatives and deliriants, but which are grouped together because of their ability to produce hallucinations (as implied by the name). 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 fervor, dreaming and even near-death experiences.
Origin of term 
The term psychedelic is derived from the Greek words ψυχή (psyche, "mind") and δηλείν (delein, "to manifest"), hence "mind-manifesting", the implication being that psychedelics can access 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 positive and non-recreational context.
General psychological effects 
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:
- First, sensory perceptions become especially brilliant and intense. Normally unnoticed aspects of the environment capture the attention; ordinary objects are seen as if for the first time and acquire new depths of significance. Esthetic responses are greatly heightened: colors seem more intense, textures richer, contours sharpened, music more emotionally profound, the spatial arrangements of objects more meaningful. People may feel keener awareness of their bodies or sense changes in the appearance and feeling of body parts. Depth perception is often heightened and perspective distorted; inanimate objects take on expressions, and synesthesia (hearing colors, seeing sounds, etc.) is common. Time may seem to slow down enormously as more and more passing events claim the attention, or it may stop entirely, giving place to an eternal present. When the eyes are closed, fantastically vivid images appear: first geometrical forms and then landscapes, buildings, animate beings, and symbolic objects.
- The emotional effects are even more profound than the perceptual ones. The drug taker becomes unusually sensitive to faces, gestures, and small changes in the environment. As everything in the field of consciousness assumes unusual importance, feelings become magnified; love, gratitude, joy, sympathy, lust, anger, pain, terror, despair, or loneliness may become overwhelming, or two seemingly incompatible feelings may be experienced at once. It is possible to feel either unusual openness and closeness to others or exaggerated distance that makes them seem like grotesque puppets or robots. The extraordinary sensations and feelings may bring on fear of losing control, paranoia, and panic, or they may cause euphoria and bliss.
- Short-term memory is usually impaired, but forgotten incidents from the remote past may be released from the unconscious and relived. Introspective reflection with a sense of deep, sometimes painful insight into oneself or the nature of humanity and the universe is common; often the experience seems somehow more real or more essential than everyday life. There are also profound changes in the sense of self: the ego may separate from the body (dissociation), or the boundary between self and environment may dissolve.
- At deeper levels, drug users may regress to childhood as they relive their memories, or they may project themselves into the series of dreamlike images before their closed eyelids and become the protagonists of symbolic dramas enacted for the mind's eye. Actions, persons, and images in this dream-world or even in the external world may become so intensely significant and metaphorically representative that they take on the character of symbols, myths, and allegories. Loss of self may be experienced as an actual death and rebirth, undergone with anguish and joy of overwhelming intensity. In some cases the culmination is a mystical ecstasy in which for an eternal moment all contradictions seem reconciled, all questions answered, all wants irrelevant or satisfied, all existence encompassed by an experience that is felt to define the ultimate reality, boundless, timeless, and ineffable.
- Some of these effects are more common than others, but none is guaranteed to occur. Many recreational users have probably never experienced the more profound and extraordinary effects, which are usually produced by larger doses, closed eyes, and deep introspection. At any dose, a great deal depends on the time, the place, and the persons involved. Each drug experience is a unique journey of exploration into the mind.
Traditional use 
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. Marijuana, one of the most widely used psychoactive drugs in the world, was thought to produce effects similar to low doses of classical psychedelics. However, the chemical structures and methods of action of cannabis and psychedelics are in no way similar to one another. Additionally, marijuana is far less likely to cause significant visual disturbances or hallucinations.
Pharmacological classes and effects 
Serotonergic or classical psychedelics (5-HT2A receptor agonists) 
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, most users report that the two 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).
Empathogen-entactogens (serotonin releasers) 
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.
Cannabinoids (CB-1 cannabinoid receptor agonists) 
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, and anxiety, potentiation of other psychedelics, and increased awareness of sound, patterns and color.
Dissociatives (NMDA antagonists) 
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. Because the effects of dissociatives tend to add a far more sedating and sensory deprived quality to the experience, they are often not considered true 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.
See also 
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- A. Weil, W. Rosen. (1993), From Chocolate To Morphine:Everything You Need To Know About Mind-Altering Drugs.New York, Houghton Mifflin Company. p. 93
- W. Davis (1996), "One River: Explorations and Discoveries in the Amazon Rain Forest". New York, Simon and Schuster, Inc. p. 120
- Grinspoon, Lester, & Bakalar, James. B. (Eds.). Psychedelic Reflections. (1983). New York: Human Sciences Press. p. 13-14 ISBN 0-89885-129-7
- D. Luke (2010), Rock art or Rorschach: Is there more to entoptics than meets the eye? Time and Mind, 3, 9-28.
- Vollenweider FX, Geyer MA. (2001) A systems model of altered consciousness: integrating natural and drug-induced psychoses. Brain Res Bull. 56: 495 - 507.
- Pomarol-Clotet E, Honey GD, Murray GK, Corlett PR, Absalom AR, Lee M, McKenna PJ, Bullmore ET, Fletcher PC. (2006) Psychological effects of ketamine in healthy volunteers. Phenomenological study. Br J Psychiatry. 189: 173 - 179.
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