Psilocin is the dephosphorylated cousin of psilocybin; together they are the main active elements in various entheogenic / hallucinogenic mushrooms -- such as Psilocybe cubensis, Panaeolus subbalteatus, and Gymnopilus spectabilis, but mainly in those mushrooms classified as Psilocybes. For other active elements, see baeocystin. Most of these mushrooms mostly contain psilocybin and only contain traces of psilocin, but the body naturally converts most of the psilocybin into psilocin after ingestion. This is about ten times as effective.
They are indole derivatives whose biosynthetic forerunner is the amino acid L-tryptophan. How much psilocybin or psilocin there is in a given mushroom varies greatly, with Psilocybe cubensis being one of the most active species; it usually contains as much as 1.3% psilocybin and up to .25-.60% psilocin per dry gram.
The effects are primarily on the brain's serotonergic systems, and are similar in quality to the effects of mescaline (or, some say, LSD). Sensory effects are the most common, especially intensified colors and "kaleidescope effects" when the eyes are closed. Paul Stamets reported that an elderly person with degraded hearing told him that "small amounts of Psilocybe cyanescens, too small to cause intoxication, had a remarkably positive effect on his hearing" (reported in Growing Gourmet and Medicinal Mushrooms). Mood alteration is also common, ranging from elation to anxiety. Feelings of paranormal occurances -- like for example, leaving the body, or travelling through time -- are not uncommon. The effects generally last about 4-10 hours, and there are no reported lingering effects or side effects.
Psilocin is less stable than psilocybin, and degrades faster. Therefore mushrooms with a higher psilocin content don't store as well or as long as mushrooms with a higher psilocybin content.
My effort at an ASCII psilocin molecular-structure:
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