The purpose of this study is to see if one or two doses of psilocybin is more effective in relieving depressive symptoms in patients with treatment-resistant depression (TRD). Researchers also want to know if a second dose of psilocybin is safe and well-tolerated. This study will see if psilocybin is effective, safe, and well-tolerated by tracking changes in depressive symptoms, suicidality, and side effects. This study will also see if a second dose of psilocybin has an effect on quality of life, functioning, cognition (thinking, reasoning, remembering), and how long depressive symptoms improve (or worsen) after psilocybin is administered.

During the past decade, there has been increased interest in the use of psilocybin as a novel treatment for mental health disorders, including treatment-resistant depression (TRD). Recent studies have suggested that psilocybin has the potential to relieve depressive symptoms when combined with psychotherapy (i.e., psilocybin-assisted psychotherapy [PAP]). Each psilocybin dosing session requires the use of extensive resources, including two specialized therapists supporting the patient for 6-8 hours per dosing session. If two doses of psilocybin prove to be more effective than a single dose of psilocybin in relieving depressive symptoms, then two doses should be the standard intervention for future trials and clinical application. However, if a second dose of psilocybin does not offer increased anti-depressant benefit from the first dose, then a second dose of psilocybin would only increase the risk of adverse side effects and cost of treatment. Therefore, the purpose of this study is to determine whether a second dose of psilocybin provides better efficacy, safety and tolerability than a single dose. The investigators hypothesize that two doses of psilocybin will be more beneficial compared to a single dose, and that there will be no significant difference between the groups (one dose versus two doses) in safety or tolerability.

The primary objective of assessing antidepressant efficacy will be evaluated by the change in the Montgomery-Åsberg Depression Rating Scale (MADRS) between baseline and Week 8. Safety and tolerability will be assessed using standardized adverse effects monitoring, in addition to close participant monitoring during the dosing day (e.g., blood pressure changes, dissociative and psychotomimetic effects, treatment-emergent manic symptoms, and suicidality). Secondary objectives include evaluating the effects of one versus two psilocybin doses on suicidality, quality of life, functioning, cognition, and duration of clinical benefits during the six month observational follow-up period. Exploratory objectives include evaluating predictors of response, such as static and dynamic clinical factors and expectancy effects, and cost-effectiveness of one versus two psilocybin doses.

Inclusion Criteria:

1. Adults 18 to 65 years old.
2. Must be deemed to have capacity to provide informed consent.
3. Must sign and date the informed consent form.
4. Stated willingness to comply with all study procedures.
5. Ability to read and communicate in English, such that their literacy and comprehension is sufficient for understanding the consent form and study questionnaires, as evaluated by study staff obtaining consent.
6. Meets DSM-5 criteria for MDD, currently experiencing a Major Depressive Episode (MDE) without psychotic features, as diagnosed by a mood disorder specialist. Diagnosis will be confirmed using the Mini-International Neuropsychiatric Interview (MINI).
7. Current MDE must be moderate to severe, as determined by a MADRS score greater than 21.
8. Have not responded to at least two trials of antidepressants at an adequate dosage and duration based on the Antidepressant Treatment History Form Short Form (ATHF-SF) with no upper limit on the number of treatment failures.
9. Ability to take oral medication.
10. Individuals who are capable of becoming pregnant: use of highly effective contraception for at least 1 months prior to screening and agreement to use such a method during study participation in addition to monthly check-ins by study staff to determine the first day of their last menstrual period;
11. Individuals who are capable of making their partner pregnant: use of condoms or other methods for the duration of study participation to ensure effective contraception with partner.
12. Individuals who are willing to taper off concomitant medications (antidepressants, antipsychotics, mood stabilizers, ketamine, esketamine, monoaminergic medicines, and stimulants) for a minimum of 1-month prior to Baseline (V2, Day 0) and whose physician confirms that it is safe for them to do so.
13. Individuals who are willing to not receive additional psychotherapy (outside of the therapy provided as part of the study) during the 8-week trial and whose physician confirms that it is safe for them to do so; however, they may continue seeing their therapist before and after this time period.
14. Individuals who have a caregiver that will be able to bring them home after treatment sessions and stay with them for a minimum of 24 hours after discharge;
15. Agreement to adhere to Lifestyle Considerations (section 4.5) throughout study duration.

Exclusion Criteria:

1. Lifetime history of mania, hypomania or psychosis as determined by clinical psychiatric assessment and the MINI.
2. Current symptoms of mania, hypomania or mixed features, as determined by the Young Mania Rating Scale (YMRS) score greater than 12.
3. Substance, cannabis, or alcohol use disorder within the past 3 months or lifetime history of hallucinogen use disorder as determined by the MINI and urine drug screen.
4. Major neurocognitive disorder, as determined by clinical assessment, including administration of the Montreal Cognitive Assessment (MoCA).
5. Have active suicidal ideation as determined by the C-SSRS and/or clinical interview (significant suicide risk is defined by suicidal ideation as endorsed by items 4 or 5 of the C-SSRS) or active suicidality requiring involuntary inpatient treatment or recent suicide attempts within the past 3 months.
6. Presence of a relative or absolute contraindication to psilocybin (within the past 12 months),, including a drug allergy, recent stroke history, uncontrolled hypertension, low or labile blood pressure, recent myocardial infarction (within the past 12 months),, cardiac arrhythmic, severe coronary artery disease, or moderate to severe renal (Glomerular Filtration Rate (GFR) less than 45ml/min/1.73 m2) or hepatic impairment (Child-Pugh B: 7 to 9 points and Child-Pugh C: 10 to 15 points).
7. Pregnant as assessed by a urine pregnancy test at Screening (V1) or individual's that intend to become pregnant during the study or are breastfeeding.
8. Treatment with another investigational drug or other intervention within 30 days of Baseline (V2).
9. Participants who will receive any form of brain stimulation (e.g., rTMS, ECT) during the trial or have within 30 days before Baseline (V2).
10. Individuals who have had changes to psychiatric medications 30 days before entering the trial, outside of as needed (PRN) medications.
11. Any DSM-5 lifetime diagnosis of a schizophrenia-spectrum disorder; obsessive- compulsive disorder, psychotic disorder (unless substance induced or due to a medical condition), bipolar I or II disorder, paranoid personality disorder, or borderline personality disorder as determined by medical history, the M.I.N.I clinical interview, and the International Personality Disorder Examination (IPDE) administered at Screening (V1).
12. Any first-degree relative with a diagnosis of schizophrenia-spectrum disorder; psychotic disorder (unless substance-induced or due to a medical condition); or bipolar I or II disorder as determined by the family medical history form and discussions with the participant.
13. Uncontrolled seizure disorder or a seizure within the past 12 months
14. Presence of baseline prolonged QTc or Torsade de Pointes as measured by the ECG or a history of long QTc syndrome or related risk factors.
15. Use of classic psychedelic drugs within the previous 6 months, including but not limited to psilocybin, psilocin, DMT, LSD, ayahuasca, mescaline, peyote, 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT).
16. Any other clinically significant physical illness including chronic infectious diseases 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 they take part in the study.

The first dose will be randomized and blinded (1mg psilocybin versus 25mg psilocybin), while the second dose will be open-label (25mg psilocybin).