Psilocin

When psilocybin is ingested, the body removes its phosphate group (dephosphorylation) to produce psilocin, which then crosses the blood-brain barrier and binds to serotonin receptors. Psilocin is the compound actually responsible for the psychedelic effects. Psilocybin itself is essentially a prodrug, a stable molecular form that the body converts into the active compound.