Psychedelic drug
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Psychedelic drugs are psychoactive drugs whose primary action is to alter the thought processes of the brain and perception of the mind. The term is derived from Greek  (psyche, "mind") and  (delein, "to manifest"), translating to "mind-manifesting." 
"The implication is that the psychedelic drugs can develop unused potentials of the human mind."  Psychedelic drugs are part of a wider class sometimes known as the hallucinogens, which also includes related substances such as dissociatives and deliriants. Unlike other psychoactive drugs such as stimulants and opioids, the psychedelics do not merely induce familiar states of mind but rather shift the locus of experiences so that they are qualitatively different from those of ordinary consciousness. The psychedelic experience is often compared to non-ordinary forms of consciousness such as trance, meditation, and dreams.
Many psychedelic drugs are thought to disable filters which keep signals unrelated to everyday functions from reaching the conscious mind. 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. This effect is sometimes referred to as mind expanding, or consciousness expanding, for the conscious mind becomes aware of  things normally inaccessible to it.
A definition 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.
Over the decades, the term has been expanded to include far more substances than originally intended. Many pharmacologists define psychedelic drugs as chemicals that have an LSD- or mescaline-like action on certain serotonin receptors. In essence, this means tryptamines and phenethylamines, as no psychedelics from other chemical families have been discovered, with the possible exception of piperazines and benzodifuranyls. Many people have applied the term psychedelic to other drugs including dissociative NMDA receptor antagonists such as PCP and ketamine, tropane deliriants such as atropine, other psychoactives such as Amanita muscaria and Salvia divinorum. However, these should be considered as separate groups. In the medical literature, they are often grouped together as hallucinogens or antidepressants (both of which are, in general, considered separate from stimulants).
The first psychedelics course listed in a university catalog is "Foundations of Psychedelic Studies" at Northern Illinois University. Taught since 1981 by Thomas B. Roberts, Ph.D., its syllabus is located at: 

# 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 peyote have reported success against alcoholism, and Mazatec practitioners routinely use psilocybin mushrooms for divination and healing. Ayahuasca, a psychotropic drug, is still used in Peru for religious festivals . Link: 

# Examples
Classic psychedelics include LSD, psilocybin (main active principle of 'magic mushrooms'), mescaline (active principle of peyote and the San Pedro cactus), LSA (Hawaiian Baby Woodrose and Morning Glory Seeds) and also Ayahuasca (known in Beatnik literature as yaje), a traditional shamanic tea brewed from plants containing dimethyltryptamine and harmine or harmaline.  Some newer synthetics such as MDMA (ecstasy), 2C-B (nexus), DOM (STP), and 5-MeO-DIPT (Foxy Methoxy) have also enjoyed some popularity. Cannabis, one of the most widely used psychoactive drugs in the world, produces effects similar to a low dose of a classic psychedelic, as well as a fairly prominent generally 'inebriated' feeling.

# Pharmacological classes and effects

## Serotonergic psychedelics (serotonin 5-HT2A receptor agonists)
This class of psychedelics includes the major hallucinogens, including tryptamine-based compounds like LSD and psilocybin, 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 distortions, 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 distortions 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 -carbolines, produce very different effects from the more standard types of psychedelics.

## Empathogens and/or entactogens (serotonin releasers)
The empathogens are phenethylamines such as MDMA, MDA, and similar drugs, the effects of which are characterized by feelings of openness, euphoria, empathy, love, and heightened self-awareness, but not by visual hallucinations. Their initial adoption by the dance club sub-culture is probably due to the enhancement of the overall social and musical experience.

## Cannabinoids (CB-1 cannabinoid receptor agonists)
The cannabinoid Tetrahydrocannabinol (THC) and related compounds are capable of activating the body's endogenous cannabinoid system. Some effects may include: general change in consciousness, mild euphoria, feelings of general well-being, relaxation or stress reduction, increased appreciation of humor, music and other art, joviality, metacognition and introspection, 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, increased awareness of patterns and color.

## Other
The effects of myristicin and elemicin (found in nutmeg) are reported are similar to that of cannabis, more so of the cannabidiol component rather than THC, but with a much longer duration, slow onset, and undesirable side-effects.
Cryogenine (Vertine) is the active constituent of sinicuichi. Although vertine has anticholinergic properties, use of sinicuichi tends to produce psychedelic effects rather than that of a deliriant (this could possibly be dose related).  The primary noted effects include auditory distortions, improved memory and relaxation.
Salvia divinorum is an atypical psychedelic. The main ingredient, Salvinorin A, is a kappa opioid receptor agonist, working on a part of the brain that deals with pain.