BACKGROUND Psilocybin is being studied for use in treatment-resistant depression. METHODS In this phase 2 double-blind trial, we randomly assigned adults with treatmentresistant depression to receive a single dose of a proprietary, synthetic formulation of psilocybin at a dose of 25 mg, 10 mg, or 1 mg (control), along with psychological support. The primary end point was the change from baseline to week 3 in the total score on the Montgomery-Åsberg Depression Rating Scale (MADRS; range, 0 to 60, with higher scores indicating more severe depression). Secondary end points included response at week 3 (≥50% decrease from baseline in the MADRS total score), remission at week 3 (MADRS total score ≤10), and sustained response at 12 weeks (meeting response criteria at week 3 and all subsequent visits). RESULTS A total of 79 participants were in the 25-mg group, 75 in the 10-mg group, and 79 in the 1-mg group. The mean MADRS total score at baseline was 32 or 33 in each group. Least-squares mean changes from baseline to week 3 in the score were -12.0 for 25 mg, -7.9 for 10 mg, and -5.4 for 1 mg; the difference between the 25-mg group and 1-mg group was -6.6 (95% confidence interval [CI], -10.2 to -2.9; P<0.001) and between the 10-mg group and 1-mg group was -2.5 (95% CI, -6.2 to 1.2; P = 0.18). In the 25-mg group, the incidences of response and remission at 3 weeks, but not sustained response at 12 weeks, were generally supportive of the primary results. Adverse events occurred in 179 of 233 participants (77%) and included headache, nausea, and dizziness. Suicidal ideation or behavior or self-injury occurred in all dose groups. CONCLUSIONS In this phase 2 trial involving participants with treatment-resistant depression, psilocybin at a single dose of 25 mg, but not 10 mg, reduced depression scores significantly more than a 1-mg dose over a period of 3 weeks but was associated with adverse effects. Larger and longer trials, including comparison with existing treatments, are required to determine the efficacy and safety of psilocybin for this disorder.