Psychedelic drug

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 related 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, and dreaming.

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, 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 religious context.

Action
Psychedelics are thought to alter the brain's ability to prevent, select or filter certain perceptions, emotions, memories and thoughts from reaching the conscious mind, an idea first introduced by Aldous Huxley in his essays The Doors of Perception (1954) and Heaven and Hell (1956), later published both in one volume. These signals are presumed to originate in several other functions of the brain, including but not limited to the senses, emotions, memories and the unconscious (or subconscious) mind.

A definition that more clearly sets apart a classic or true psychedelic is offered by Lester Grinspoon: “a psychedelic drug is one which has small likelihood of causing physical addiction, craving, major physiological disturbances, delirium, disorientation, or amnesia, produces thought, mood, and perceptual changes otherwise rarely experienced except perhaps in dreams, contemplative and religious exaltation, flashes of vivid involuntary memory and acute psychoses”. The word "psychedelic" is often used interchangeably with "psychotomimetic" and "hallucinogen." Hoffer & Osmond considered "psychedelic" and "psychotomimetic" to refer to a particular kinds of drug reaction. They stated that "for these reasons it seems appropriate to continue using the term hallucinogens for a variety of substances which can produce reactions which may be psychotomimetic, psychedelic, or delirient, depending upon many other factors."

Hollister's definition of hallucinogen included drugs "... that produce changes in thought, mood, and perception with little memory or intellectual impairment, and that produce little stupor, narcosis, or excessive stimulation, minimal autonomic side effects, and that are nonaddicting."

The term psychedelic has been used to refer to a pharmacologically diverse set of drugs, including classical hallucinogens such as LSD, mescaline, and psilocybin; entactogens such as MDMA, MDA and 2CB; cannabinoids such as THC; dissociative hallucinogens such as phencyclidine, dextromethorphan, and ketamine; tropane deliriants such as atropine, and other psychoactives such as Amanita muscaria and Salvia divinorum. However, according to some, only the classical hallucinogens such as LSD, mescaline, psilocybin or DMT, can be considered true psychedelics; in particular, these and other similar examples (i.e. agonists selective for the 5HT2A/"psychedelic" serotonin receptor), tend not to cause dependence.

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.

Examples
Classic psychedelics include LSD ("Acid", "Lucy"), a semi-synthetic psychedelic derived from ergot and discovered by the late Albert Hoffman in 1938 (the psychoactive properties were not, however, established until 1943 - see History of LSD), psilocybin and psilocin (active constituents of Psilocybe mushrooms, also known as "Magic Mushrooms" or "Shrooms"), mescaline (active constituent of Peyote, San Pedro, and Peruvian Torch cacti), ergolines (active constituents of Hawaiian Baby Woodrose, Morning glory, Turbina corymbosa, and ergot) (LSD is also considered to be an ergoline) and dimethyltryptamine (DMT) (an endogenous tryptamine and primary active constituent of Ayahuasca, a traditional shamanic tea brewed from plants containing DMT and harmala alkaloids). A few newer synthetics such as MDMA ("Ecstasy"), 2C-B, DOM, and 5-MeO-DIPT have also enjoyed some popularity. Cannabis, one of the most widely used psychoactive drugs in the world, produces effects similar to low doses of classic psychedelics, though at higher doses or in susceptible individuals it can be quite psychedelic, depending on the strain.

Serotonergic psychedelics (serotonin 5-HT2A receptor agonists)
This class of psychedelics includes the classical hallucinogens, including the ergolines like LSD and LSA, tryptamine-based compounds like psilocybin and DMT, and phenethylamine-based compounds like mescaline and 2C-B. Many of the tryptamines and phenethylamines 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. Some drugs, such as the β-carbolines, produce very different effects from the more standard types of psychedelics.

Empathogen-entactogens (serotonin releasers)
The empathogen-entactogens are phenethylamines such as MDMA (Ecstasy), MDA, and MDEA, among others. Their effects are characterized by feelings of openness, euphoria, empathy, love, heightened self-awareness, and by mild visual distortions. 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. This gives the user, subjectively, the "best of both worlds".

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, 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. They are more similar to the above categories as dose increases.

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.

Other
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. This explains, to an extent, the majority of S. divinorum experiences which are reported as negative. 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 i.e. a pane of glass or a pencil. It is also unusual as it is a diterpenoid as opposed to the general alkaloid standard for psychedelics.