The goal of this clinical trial is to learn if a single dose of psilocybin (5mg Vs 10mg Vs 25mg) alongside psychotherapy is safe and can help treat treatment resistant depression (TRD) with co-occurring substance use disorder (SUD) in veterans and first responders. We seek to answer: * Whether 5mgs, 10mgs and 25mgs of psilocybin are safe in individuals with co-occurring TRD and SUD * Whether psilocybin assisted psychotherapy will reduce substance use severity and depression symptoms * What neurobiological processes are associated with the effects of psilocybin assisted psychotherapy. The researchers will compare the effects of a single dose of psilocybin (either 5mgs or 10mgs or 25mg) alongside psychotherapy on substance use severity and depression symptoms over six weeks in veterans and first responders with TRD and co-occurring SUD. In this 14-week study, participants will: * Visit the clinic for two intake sessions * Complete seven psychotherapy sessions. This will include three sessions before psilocybin administration, an 8 to 10 hour dosing session, and three sessions following psilocybin administration * Complete short, repeated daily assessments for six weeks, in total, before and after psilocybin administration * Complete two brain scans before and after psilocybin administration
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percentage number of participants with Adverse Events (AEs)
Timeframe: Weekly for 12-weeks and once during a 60-day follow-up
Change in % number of positive urines
Timeframe: Weekly for 12-weeks
Change in % number of days spent using substances
Timeframe: Weekly for 12-weeks TLFB and 5 times per day in weeks 1, 2, 5, 6, 10 & 11 (brief items)
Change in severity of depression symptoms
Timeframe: Weekly for 12-weeks QIDS and 5 times per day in weeks 1, 2, 5, 6, 10 & 11 for PANAS
Changes in plasma brain-derived neurotrophic factor (BDNF) during stress exposure
Timeframe: Change from baseline (week 2) to post psilocybin administration (week 5)
Change in resting brain functional connectivity of the default mode network (DMN).
Timeframe: Change from baseline (week 2) to post psilocybin administration (week 5)