Psilocybin is a psychodysleptic substance of natural origin present in the Psilocybe type mushrooms. Psychodysleptics are psychoactive substances which strongly affect the perception, mood and cognitive processes of a human. The mind-altering operation of psilocybin is based on stimulating the serotoninergic receptors, which leads to the increase in the concentration of serotonin in the brain and contributes to the intensification of the sensorimotor activity and perception. The effect of the operation of psilocybin on the human body are various changes in behaviour – frequent euphoria, cheerfulness, the feeling of lightness and unity with the surrounding world. Additionally, psilocybin causes perception modification not imitating reality, often mistaken for hallucinations. The operation of psilocybin may be compared to the operation of LSD (lysergic acid diethylamide), yet many times weaker. Psilocybin may be found on the list of intoxicants in the I-P group, namely substances with no medical applications and with a high potential of overusing, which are excluded from the pharmaceutical trade and may be used solely for scientific purposes. However, the properties of psilocybin make it possible to consider its use in medicine. Ailments, in which the use of this substance brings measurable effects, include depression. Studies conducted on volunteers show that psilocybin may be a good alternative to the currently available antidepressants – its effectiveness was observed and very low toxicity confirmed.