This open-label pilot study seeks to investigate the safety and efficacy of psilocybin in persons with chronic anorexia nervosa (AN). Psilocybin has previously been demonstrated to decrease depression and anxiety and increase long-term positive behavior change in other populations. The investigators seek to determine whether similar changes can be safely produced in people with AN when psilocybin is administered in a supportive setting with close follow-up. The investigators' primary hypotheses are that psilocybin is safe to administer in people with AN, that it will reduce measures of anxiety and depression, and that it will lead to increased quality of life. The investigators will also assess a number of exploratory measures related to eating disorder pathophysiology.

General Inclusion Criteria:

• Currently meet criteria for AN restricting subtype by Diagnostic and Statistical Manual 5th edition (DSM-5) criteria, and have a history of AN for at least 3 years prior to screening
• Have at least one prior attempt at treatment
• 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 pregnancy test, urine toxicology screen).
• Agree to refrain from using any psychoactive drugs, including alcoholic beverages and nicotine, within 24 hours of each drug administration. The exception is caffeine.

General 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 childbearing 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, heart valve, or transient ischemic attack in the past year. Resting heart rate may be no less than 50 beats per minute unless cleared by a cardiologist.
• Epilepsy with history of seizures
• Type 1 diabetes
• First degree relative with schizophrenia spectrum or other psychotic disorders (except substance/medication-induced or due to another medical condition), or Bipolar I Disorder
• Has a psychiatric condition judged to be incompatible with establishment of rapport or safe exposure to psilocybin