Effects of Psilocybin in Obsessive Compulsive Disorder

Inclusion Criteria: * Have given written informed consent * Currently meet criteria for a DSM-5 diagnosis of OCD and report a history of OCD for at least 1 year prior to screening * Have a Y-BOCS score of 18 or more * Have at least one prior attempt at treatment, either ERP or pharmacotherapy * No antidepressant medications for approximately five half-lives prior to acceptance in treatment phase of study * Women who are of childbearing potential and sexually active who are not practicing an effective means of birth control must agree to practice an effective means of birth control throughout the duration of the study * Be judged by study team clinicians to be at low risk for suicidality * Concurrent psychotherapy is allowed if the type and frequency of the therapy has been stable for at least two months prior to screening and is expected to remain stable during participation in the study * 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 (CBC, CMP, urine beta-HCG, urine toxicology screen) * 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 n…