Psilocybin-Assisted Therapy in Treatment-Resistant Depression

The goal of this clinical trial is to test how well psilocybin-assisted therapy works in treating people with depression. The main questions this study aims to answer are: * Does psilocybin with assisted therapy help improve symptoms for people with depression? * How long do the effects of this treatment last? Participants will: * Take part in a couple of screening and preparation visits. * Be given psilocybin in one or two treatment sessions. * Attend a series of follow-up sessions over the following year. * Complete forms and surveys to test how their symptoms have changed and what they thought of their experience. Researchers will also compare whether one treatment or two treatments help improve symptoms more for participants.

Major depressive disorder (MDD) ranks fourth in global disease burden and has significant morbidity, mortality, societal and financial costs. However, few adequate and effective treatments exist with 60% of MDD patients not responding sufficiently to an initial oral antidepressant treatment. These patients who experience treatment resistant depression (TRD), defined as an intolerance or lack of response to two antidepressants of different classes, have limited treatment options beyond the antidepressant treatments that often yield insufficient results or relapse. Psilocybin, a novel treatment, has been found to relieve symptoms of TRD, but there are limited studies on specific dosing and long term treatment follow-up. In this study, the investigators will look closer at the effectiveness of one treatment with psilocybin versus two treatments with psilocybin, as well as the long term effectiveness over the first 12 months after treatment.