• 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

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..
• * 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