Effects of Psilocybin in Major Depressive Disorder

Inclusion Criteria: * 21 to 75 years old * Have given written informed consent * Have at least a high-school level of education or equivalent (e.g. GED). * Have a confirmed Diagnostic Statistical Manual (DSM-5) diagnosis of Major Depressive Disorder and currently experiencing a major depressive episode. * No antidepressant medication for at least 2 weeks (4 weeks for fluoxetine) prior to enrollment. * 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 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 any psychoactive drugs, including alcoholic beverages and nicotine, within 24 hours of each drug administration. The exception is caffeine. Participants will be required to be non-smokers. * Agree not to take any Pro re nata (PRN) medications on the mornings of drug sessions * Agree not to take sildenafil (Viagra®), tadalafil, or similar medications within 72 hours of each drug administration. * Agree that for one week before each drug session, he/she will refrain from taking any nonprescription medication, nutritional supplement, or herbal supplement except when approved by the study investigators. Exceptions will be evaluated by the study investigators and will include acetaminophen, non-steroidal anti-inflammatory drugs, and common doses of vitamins and minerals. Exclusion Criteria: * Women who are pregnant (as indicated by a positive urine pregnancy test assessed at intake and before each drug session) or nursing; women who are of child-bearing potential and sexually active who are not practicing an effective means of birth control. * Cardiovascular conditions: coronary artery disease, stroke, angina, uncontrolled hypertension, a clinically significant ECG abnormality (e.g., atrial fibrilation), prolonged QTc interval (i.e., QTc \> 450 msec), artificial heart valve, or Transient Ischemic Attack (TIA) in the past year * Epilepsy with history of seizures * Insulin-dependent diabetes; if taking oral hypoglycemic agent, then no history of hypoglycemia * Currently taking psychoactive prescription medication on a regular (e.g., daily) basis * Currently taking on a regular (e.g., daily) basis any medications having a primary centrally-acting serotonergic effect, including Mono-Amine Oxidase Inhibitors (MAOIs). For individuals who have intermittent or PRN use of such medications, psilocybin sessions will not be conducted until at least 5 half-lives of the agent have elapsed after the last dose. * More than 25% outside the upper or lower range of ideal body weight according to Metropolitan Life height and weight table Psychiatric Exclusion Criteria: * Current antidepressant use * Current or past history of meeting DSM-5 criteria for schizophrenia spectrum or other psychotic disorders (except substance/medication-induced or due to another medical condition), or Bipolar I or II Disorder * Current or history within one year of meeting DSM-5 criteria for a moderate or severe alcohol, tobacco, or other drug use disorder (excluding caffeine) * Have a first or second-degree relative with schizophrenia spectrum or other psychotic disorders (except substance/medication-induced or due to another medical condition), or Bipolar I or II Disorder * Has failed to respond to electroconvulsive therapy during the current major depressive episode * Has a psychiatric condition judged to be incompatible with establishment of rapport or safe exposure to psilocybin Additional Magnetic Resonance Imaging (MRI) Exclusion Criteria: * Head trauma * Claustrophobia incompatible with scanning * Cardiac pacemaker * Implanted cardiac defibrillator * Aneurysm brain clip * Inner ear implant * Prior history as a metal worker and/or certain metallic objects in the body * History of clinically significant vertigo, seizure disorder, middle ear disorder, or double vision * Poor vision not adequately corrected (in order to complete emotional processing task)