Investigating the Therapeutic Effects of Psilocybin in Treatment-Resistant Post-Traumatic Stress Disorder

Inclusion Criteria: \- After signing and dating the informed consent documents, subject eligibility will be assessed. Subjects must meet the following criteria to be eligible for enrollment into the study. 1. Subjects must be ≥18 and ≤70 years of age. 2. Subjects must meet the Diagnostic \& Statistical Manual of Mental Disorders - Version V (DSM-V) criteria for TR-PTSD. 3. Subjects must have treatment-resistant PTSD symptoms, defined as a CAPS score of ≥30 (signifying moderate to severe symptoms) following at least 3 months of prior SSRI or serotonin-norepinephrine reuptake inhibitor (SNRI) treatment in addition to at least 4 months of psychotherapy (adapted from Mithoefer et al, 2011). 4. Subjects must be able to communicate in English. Exclusion Criteria: * Subjects meeting any of the following criteria will not be eligible for participation in the study: 1. Pregnant individuals and those of childbearing age not using effective contraception (e.g., oral contraceptive pill, injection, implant, patch, vaginal ring, intrauterine coil, intrauterine device, tubal ligation, or barrier method). 2. Uncontrolled hypertension or BP ≥140/90 mmHg over 2 days, with at least 4 BP assessments completed. 3. In the clinical judgement of the investigator, any hazard-posing medical, emotional, or significant character disorder or condition rendering unsuitability for the study. For example, poorly controlled diabetes, severe cardiovascular disease, seizure disorders, sleep apnea disorders (suspected or ineffectively treated), untrustworthiness, suicidality, etc. 4. Any use of methamphetamines or any injection drug abuse in the past 30 days and/or a positive test for drugs of abuse (e.g., cocaine, amphetamines, opiates, benzodiazepines, etc.). 5. Any other significant substance use disorder that may interfere with study objectives including consuming \>5 cups of caffeinated coffee a day or inability, without discomfort, to refrain from smoking cigarettes or cannabis, or consuming alcohol for 7 hours. 6. Blood draw or needle phobia. 7. Suicidal attempt or active ideation deemed to present risk of suicide as judged by study clinical staff in past 30 days.. 8. BMI \<14 or \>42 or the Qualified investigator deems the patient sufficiently healthy to participate. 9. Current or previously diagnosed schizophrenia spectrum or other psychotic disorders, including schizophrenia, schizoaffective disorder, schizotypal disorder, schizophreniform disorder or brief psychotic disorder; current or previous history of bipolar disorder, or obsessive-compulsive disorder. 10. Any uncontrolled eating disorder (e.g., purging or anorexia or worsening of directionally undesirable weight change of 5 kg in past 30 days). 11. Subjects with a diagnosis of DSM-5 personality disorder which has a major impact on the subject's current psychiatric status 12. Use of any investigational drug, hallucinogen, or ketamine/esketamine within the past 30 days, or plan to use during the study.