This invention relates to a formulation of psilocybin which addresses problems of uniformly dispersing the psilocybin active in the excipients. The formulation comprises psilocybin, a silicified filler, preferably silicified microcrystalline cellulose, and optionally a disintegrant, glidant and lubricant. The preferred disintegrant, glidant and lubricant are sodium starch glycolate, colloidal silicon dioxide and sodium stearyl fumurate respectively. The psilocybin is preferably in a crystalline form and may be polymorph A, polymorph A’, polymorph B or hydrate A. The preferred particle size range for the silicified microcrystalline cellulose excipient is 90-150 microns. The formulation may be an oral dosage form, particularly a tablet.