The primary aim of this study is to assess the safety and tolerability of one 25 mg dose of psilocybin in participants with anorexia nervosa based on adverse events (AEs), changes in vital signs, electrocardiograms (ECGs) and clinical laboratory tests. The secondary objectives are to explore the efficacy of a single 25 mg dose of psilocybin on eating disorder symptoms and behaviors, body image, anxiety, food related obsessions and rituals, and body weight.

Because there are no proven treatments that normalize core symptoms in adult anorexia nervosa, a disorder with high chronicity, many individuals seek out alternative approaches to care. Recent evidence has suggested that anxiety, obsessive compulsive disorder, and diminished reward or motivation play key roles in the development and maintenance of dysfunctional eating, and poor outcome. In recent years, a growing number of studies have demonstrated the safety and preliminary efficacy of psilocybin in clinical trials for a range of psychiatric illnesses including treatment resistant depression, obsessive compulsive disorder, addiction, and anxiety. Psilocybin may represent a promising new treatment for anorexia nervosa. However, no studies have tested psilocybin in this eating disorder population. Accordingly, this study aims to establish the safety, tolerability and dosing of psilocybin in adult patients with anorexia nervosa, as well as gather pilot data on possible efficacy.

For this study, the investigators will recruit adults who currently have a DSM-V diagnosis of anorexia nervosa. Participants will undergo medical and psychological screening and those who are deemed eligible will partake in a maximum of 7 study visits, lasting from 4-8 weeks. On dosing day, participants will receive a single 25 mg dose of psilocybin along with psychotherapeutic support, which includes preparation and integration sessions surrounding the experience. There will be a follow-up period of one month following the psilocybin session during which a range of psychological measures (questionnaires and interviews) will be collected.

Inclusion Criteria:

1. 18 to 40 years of age at Screening
2. Current diagnosis of Anorexia Nervosa (informed by DSM 5) based on medical records, clinical assessment, weight, and documented completion of the version 7.0.2 Mini International Neuropsychiatric Interview (MINI)
3. Agree for the study team to maintain contact with their primary care team for the duration of the study.
4. Ability to complete all protocol required assessment tools without any assistance or alteration to the copyrighted assessments, and to comply with all study visits.

Exclusion Criteria:

Medical exclusion criteria will be determined during the Screening Period and Baseline. Exclusion assessments that will be rechecked on the day of dosing are marked with an Asterix.

1. BMI < 16 kg/m2 *
2. Medical instability as indicated by significant (>3kg) weight loss during the screening period, orthostatic heart rate and blood pressure *
3. Women who are pregnant, nursing, or planning a pregnancy in the near future. Male and female participants who are sexually active must agree to use a highly effective contraceptive method throughout their participation in the study. Women of child bearing potential must have a negative urine pregnancy test at Screening visits and Baseline, and psilocybin dosing session days *
4. Cardiovascular conditions: recent stroke (<1 year from signing of ICF), recent myocardial infarction (<1 year from signing of ICF), uncontrolled hypertension (blood pressure >140/90 mmHg) or clinically significant arrhythmia within 1 year of signing the ICF.
5. Uncontrolled or insulin-dependent diabetes.
6. Seizure disorder.
7. Use of psychedelics, including psilocybin, within one year prior to Screening assessment
8. Positive urine drug screen for illicit drugs or drugs of abuse in the Screening Period and Baseline and psilocybin dosing days. Any positive urine drug test will be reviewed with participants to determine the pattern of use and eligibility will be determined at the investigator's discretion *
9. Current enrolment in any investigational drug or device study or participation in such within 30 days prior to Screening
10. Abnormal and clinically significant results on the physical examination, vital signs, ECG, or laboratory tests at Screening, such as liver function tests (LFTs) three times greater than the upper limit of normal, reduced glomerular filtration rate (GFR) and elevated creatinin two times of upper limit of normal
11. Any other clinically significant cardiovascular, pulmonary, gastrointestinal, hepatic, renal or any other major concurrent illness that, in the opinion of the investigator, may interfere with the interpretation of the study results or constitute a health risk for the participant if he/she takes part in the study
12. Non-English speakers
13. Current or past history of schizophrenia, psychotic disorder, bipolar disorder, significant history of mania, delusional disorder, paranoid personality disorder, schizoaffective disorder, or borderline personality disorder as assessed by medical history and a structured clinical interview
14. McLean Screening Instrument for Borderline Personality Disorder >7 at Screening
15. Currently taking a serotonergic medication. All serotonergic medication must be discontinued at least two weeks prior to Baseline.
16. Current (within the last year) alcohol or substance use disorder as informed by DSM-5 at Screening
17. Significant suicide risk as defined by (1) suicidal ideation as endorsed on items 4 or 5 on the Colombia-Suicide Severity Rating Scale (C-SSRS) within the past year, at Screening or at Baseline, or; (2) suicidal behaviors within the past year, or; (3) clinical assessment of significant suicidal risk during subject interview (pre-treatment Baseline sessions).
18. Other personal circumstances and behavior judged to be incompatible with establishment of rapport or safe exposure to psilocybin, including exposure to psilocybin within the past year and use of psychedelics, such as ayahuasca, during the current episode.