This is an open-label trial evaluating feasibility, tolerability, safety and efficacy of psilocybin assisted cognitive processing therapy for chronic Posttraumatic Stress Disorder (PTSD).

Current front-line treatments for Posttraumatic stress disorder (PTSD) are ineffective for up to half of patients, with serious medical and societal consequences. It is imperative to improve the efficacy of front-line treatment options, such as cognitive processing therapy (CPT). CPT is an effective treatment for PTSD, including when delivered intensively (i.e., multiple sessions over 7 days). However, a substantial proportion of patients continue to meet criteria for PTSD or have residual PTSD symptoms post-treatment. Psilocybin-assisted CPT may be a potential solution, as preliminary evidence supports the potential of psilocybin to alleviate symptoms of PTSD.

Fifteen participants will receive a single dose of psilocybin 25mg combined with 12 sessions of massed CPT, and 2 psychotherapy sessions related to psilocybin over 7 days. Participants will complete clinician-administered scales, self-reported mental health questionnaires, and use a wearable device. After the 1-week interventional period, participants will enter a 12-weeks follow-up period.

Inclusion Criteria:

1. Meet Diagnostic and Statistical Manual-5th edition (DSM-5) criteria for current PTSD with a duration of 6 months or longer assessed by study psychiatrist;
2. Have a Clinician-Administered PTSD Scale for DSM-5 (CAPS-5) score of 50 or higher, indicating moderate to severe PTSD symptoms;
3. Are willing to refrain from taking any psychiatric medications during the study period.

Exclusion Criteria:

1. Are pregnant or nursing, or are women of child bearing potential who are not practicing an effective means of birth control;
2. Have a history of or a current primary diagnosis of psychotic disorder, schizophrenia, delusional disorder, borderline personality disorder, schizoaffective disorder, bipolar disorder or, dissociative identity disorder;
3. Have evidence or history of coronary artery disease or cerebral or peripheral vascular disease, hepatic disease with abnormal liver enzymes, or any other medical disorder judged by the investigator to significantly increase the risk of psilocybin administration;
4. Have hypertension using the standard criteria of the American Heart Association (values of 140/90 or higher assessed on three separate occasions;
5. History of seizure disorder;
6. Uncontrolled insulin-dependent diabetes;
7. Recent stroke, intracranial or subarachnoid hemorrhage (< 1 year from signing of informed consent form [ICF]), recent myocardial infarction (< 1 year from signing of ICF), clinically significant arrhythmia (< 1 year from signing of ICF);
8. Have liver disease with the exception of asymptomatic subjects with Hepatitis C who have previously undergone evaluation and successful treatment;
9. Lifetime history of substance-induced psychosis;
10. Lifetime history of substance use disorder with a hallucinogen;
11. History of alcohol use disorder in the past 3 months.