A Study of Psilocybin for PTSD

Inclusion criteria: * Be at least 21 years old * Have given written informed consent * Have a confirmed DSM-5 diagnosis of Post-Traumatic Stress Disorder with symptom duration \>= 6 months * Have a baseline CAPS-5 score of \>=35 * Currently taking a serotonin reuptake inhibitor (SSRI or SNRI) at a stable dose for at least 3 months. * Be judged by study team clinicians to be at low acute risk for suicidality * Be 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 * Agree to consume approximately the same amount of caffeine-containing beverage (e.g., coffee, tea) that he/she consumes on a usual morning, before arriving at the research unit on the mornings of drug session days. If the participant does not routinely consume caffeinated beverages, he/she must agree not to do so on session days. * Agree to refrain from using alcohol, anxiolytics, stimulants, sedatives or hypnotics, opioids, dissociatives, cannabinoids, or other unapproved substances within 24 hours of psilocybin administration. Caffeine and nicotine are exceptions. * Agree not to take any as needed (PRN) medications on the mornings of drug sessions * Agree to stop taking 5HT2A antagonist medications at least 5 half-lives before psilocybin dosing. * Agree not to take sildenafil (Viagra®), tadalafil, or similar medications within 72 hours of ea…