A tryptamine alkaloid, isolated from various genera of fungi including the genus Psilocybe, with hallucinogenic, anxiolytic, and psychoactive activities. In vivo, psilocybine is rapidly dephosphorylated into the active compound psilocin, which activates serotonin 2A (5-HT2A) receptors in the central nervous system (CNS), mimicking the effects of serotonin. or A substance being studied in the treatment of anxiety or depression in patients with advanced cancer. It is taken from the mushroom Psilocybe mexicana. Psilocybine acts on the brain to cause hallucinations (sights, sounds, smells, tastes, or touches that a person believes to be real but are not real). Also called psilocybin.