In this phase 2 double-blind trial, adults with treatment-resistant depression were randomly assigned to receive psychological support and a single dose of a proprietary, synthetic form of psilocybin at a dose of 25 mg, 10 mg, or 1 mg (control). The Montgomery-Sberg Depression Rating Scale (MADRS) total score change from baseline to week three was the main outcome measure. Secondary endpoints included remission at week 3, response at week 3, and sustained response at 12 weeks.