Psilocybin for the Treatment of Veterans With Post-Traumatic Stress Disorder

In order to be eligible to participate in this study, an individual must meet all of the following criteria: * A US military Veteran * 21 to 64 years old * Have at least a high-school level of education or equivalent. * Have a current DSM-5 diagnosis of Post-Traumatic Stress Disorder * Have a CAPS-5 total severity score of ≥35 at baseline * No antidepressant medications prior to enrollment * Be judged by study team clinicians to be at low risk for suicidality * Be medically stable * Have limited lifetime use of hallucinogens General medical exclusion criteria: * Women who are pregnant * Cardiovascular conditions * Epilepsy with history of seizures * Insulin-dependent diabetes * Currently taking psychoactive prescription medication * Currently taking on a regular (e.g., daily) basis any medications having a primary centrally-acting serotonergic effect, including Monoamine oxidase inhibitors.. Psychiatric Exclusion Criteria: * Current or past history of schizophrenia or other psychotic disorders or Bipolar I or II Disorder * Current or history within one year of a moderate or severe alcohol, tobacco, or other drug use disorder * Have a first or second-degree relative with schizophrenia spectrum or other psychotic disorders or Bipolar I or II Disorder * Has a psychiatric condition which precludes the establishment of therapeutic rapport * History of a medically significant suicide attempt * Current antidepressant use