Psilocybin Assisted Psychotherapy for Treatment Resistant Depression and Co-occurring Substance Use Disorder

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