Evaluating the Effect of Length of Time on Selective Serotonin Reuptake Inhibitors (SSRIs) on the Response to Psilocybin-assisted Therapy in Individuals With Mild-moderate Major Depressive Disorder (MDD)

This study is an open-label, single-arm, within-subjects design in individuals with mild-moderate Major Depressive Disorder (MDD). All participants will receive a single dose of 25mg of psilocybin in a therapeutic setting. In order to investigate the effects of length of time on SSRI therapy, 30 participants with varying lengths of time on SSRI therapy will be enrolled, stratified into four groups: * Group 1: ≤ 1 year * Group 2: 1 to ≤ 5 years * Group 3: 5 to ≤ 10 years * Group 4: \> 10 years The majority of clinical investigations with psilocybin to date either exclude participants on SSRIs or taper them off SSRIs prior to psilocybin administration. While evidence derived from the use of larger doses of psilocybin suggests that its predominately serotonergic effects are safe when administered in controlled settings, research investigating the effects of psilocybin with individuals taking SSRIs is lacking, despite the prevalent and chronic use of SSRIs in individuals with depression. The aim of this study is to investigate the effect of length of time on SSRIs on psilocybin-assisted therapy response in individuals with MDD. Specifically, this feasibility study investigates participants who undergo a single-dose of psilocybin (25mg) in combination with pre- and post-dose therapy sessions. The follow-up period in the present study is 12 weeks (3 months).