Safety and Tolerability of Psilocybin in Post-Traumatic Stress Disorder

Inclusion Criteria: * 18 to 80 years old. Participants up to 80 years old who otherwise meet safety criteria will be included to ensure generalizability to the broader clinical population of people with PTSD. Prior work (Griffiths et al. 2016) by our research team administering psilocybin to older cancer patients using the same upper age limit in the inclusion/exclusion criteria (i.e., 80 years old) found a comparable risk profile for psilocybin to research conducted with younger patients. Although it is possible that older patients experience diminished serotonin 2A (5-HT2A) receptor expression resulting in a lower/less intense response to psilocybin, this information will be important to gather in this Phase I context. * Have given written informed consent * Read, write, and speak English * At Screening, meet Diagnostic and Statistical Manual-5th edition (DSM-5) criteria for current PTSD with a symptom duration of 6 months or longer according to the Clinician-Administered PTSD Scale for DSM-5 * Able to complete the study measures * Previously sought treatment for PTSD (e.g., prolonged exposure therapy, cognitive processing therapy, sertraline, paroxetine) * Be otherwise medically stable as determined by screening for medical problems via a personal interview, a medical questionnaire, a physical examination, an electrocardiogram (ECG), and routine medical blood and urinalysis laboratory tests (Complete Blood Count, Comprehensive Metabolic Panel, urine beta-human chorionic gonadotropin, urine toxicology screen). Exclusion Criteria: * Current physical dependence (as evidenced by self-reported withdrawal symptoms) on a drug other than caffeine or nicotine * Seizure disorder * Receiving current treatment for PTSD * Cardiovascular conditions: angina, a clinically significant ECG abnormality (e.g. atrial fibrillation or corrected QT interval \>450msec), transient ischemic attack (TIA) in the last 6 months, stroke, or uncontrolled hypertension with resting blood pressure systolic \>150 or diastolic \>95. * Recent (\<1year) intracranial or subarachnoid hemorrhage, ischemic stroke, TIA * Pulmonary disease: chronic obstructive pulmonary disease, active asthma (inhaler use in last 6 months) * Diabetes mellitus treated with insulin or oral hypoglycemic agents * Current suicidal ideation or suicidality * Current engagement in evidence-based PTSD therapy/treatment (prior to psilocybin session) * Women: Pregnancy (pregnancy tests will be conducted for women during screen and prior to experimental sessions). * Current or past history of meeting DSM-5 criteria for Schizophrenia, Psychotic Disorder (unless substance-induced or due to a medical condition), or Bipolar I Disorder. * Currently taking efavirenz or serotonin-acting dietary supplements (e.g., 5-hydroxy- tryptophan, St. John's wort). * Currently taking antidepressants of any drug class, antipsychotics, or monoamine oxidase inhibitors. * Recent (within past 12 months) or extensive history of hallucinogen use (\>20 lifetime uses). * Moderate or severe DSM-5 Substance Use Disorder in the past five years (excluding tobacco and caffeine) * Family (i.e., 1st degree relative) history of Schizophrenia, Psychotic Disorder (unless substance-induced or due to a medical condition), or Bipolar I Disorder. * For the final (5th) dose sequence (35, 40, and 45 mg) participants that weigh less than 50 kg