Participant drop-out rates will be calculated to determine the feasibility of psilocybin in adults with treatment-resistant depression.

Feasibility will be judged based on change in Columbia Suicide Severity Rating Scale (CSSRS) scores. The CSSRS evaluates suicidal ideation and behaviour. The suicidal ideation score ranges from 0 (no ideation) to 5 (active suicidal ideation with specific plan and intent). Suicidal ideation intensity score ranges from 0 (no ideation) to 25 (most severe). The presence of suicidal behaviour is rated as a binary response; the lethality of actual attempts are rated on a scale of 0 (no or very minor physical damage) to 5 (death) and the potential lethality of actual attempts are rated on a scale of 0 (behaviour not likely to result in injury) to 2 (behaviour likely to result in death despite available medical care).

Feasibility will be judged based on the percentage of participants experiencing serious adverse events and the percentage of adverse events resolving within 48 hours of each dose administration.