The Safety and Tolerability of COMP360 in Participants With Post-traumatic Stress Disorder

Key Inclusion Criteria: * Meet DSM-5 criteria for current PTSD resulting from a trauma experienced during adulthood measured via the PCL-5 in combination with the LEC-5 at screening * Meet DSM-5 criteria for current PTSD resulting from a trauma experienced during adulthood as assessed by the CAPS, with a minimum score of 25 at baseline * Able to identify a next of kin who is willing and able to be reached by the investigators in case of emergency * Have successfully discontinued all prohibited medications at least two weeks prior to baseline visit. For fluoxetine (Prozac), immediate cessation at screening period visit 1a followed by at least four weeks of run-in will be required prior to baseline Key Exclusion Criteria: * Current or past history of schizophrenia, schizoaffective disorder or any other form of psychotic disorder, obsessive compulsive disorder, personality disorders, bipolar disorder, or any other significant disorder as assessed by clinician judgement and a structured clinical interview (MINI 7.0.2) * Diagnosis of complex PTSD according to clinician judgement * Borderline Personality Disorder as demonstrated by both the McLean Screening Instrument for Borderline Personality Disorder (MSI- BPD) score ≥ 7 and clinical confirmation of diagnosis by the study clinician and Medical Monitor * Significant suicide risk as defined by (1) suicidal ideation as endorsed on items 4 or 5 on the C-SSRS within the past year, during screening or at baseline, or; (2) suicidal behaviours within the past year, or; (3) history of serious suicide attempt that required a rescuing medical intervention, or; (4) clinical assessment of significant suicidal risk during participant interview * Current (within the last year) alcohol or substance use disorder as informed by DSM-5 assessed via the MINI 7.0.2 at screening * Other personal circumstances and behaviour judged to be incompatible with establishment of rapport or safe exposure to psilocybin * Exposure to 3,4-methylenedioxymethamphetamine (MDMA), psilocybin, or any other psychedelics, such as ayahuasca, mescaline, lysergic acid diethylamide (LSD), or peyote in the past year * Primary diagnosis of major depressive disorder within 6 months of study entry * Exposure to a traumatic experience in the past 3 months * Significant childhood physical or sexual abuse based on clinician judgment with the use of CTQ * Enrolment in a psychological therapy programme that will not remain stable for the duration of the study. Psychological therapies cannot have been initiated within 21 days of baseline