Psilocybe Cubensis Mushrooms With or Without Fluoxetine for Refractory Depression: a Phase 2a Pilot Randomized Clinical Trial (COGUNILA)

Inclusion Criteria Age: ≥25 and \<65 years. Diagnosis: Current Major Depressive Disorder (MDD), moderate to severe, per DSM-5-TR, confirmed with SCID-5.

Baseline severity: MADRS ≥20 at baseline (reassessed at the pre-dose visit to confirm ongoing eligibility).

Partial Response in the current episode (PRD): ≥1 adequate antidepressant trial in this episode (therapeutic dose for ≥6-12 weeks, adherence ≥80%) with \<50% symptom reduction or clinically significant residual symptoms.

Clinical stability and ability to provide informed consent; willingness to comply with all study procedures (preparation, dosing session, integration, and follow-ups).

Contraception: For participants with reproductive potential, negative pregnancy test and agreement to use effective contraception during the study.

Exclusion Criteria Psychiatric disorders: Bipolar I/II disorder, any psychotic disorder, or current MDD with psychotic features; first-degree family history of psychotic or bipolar disorder.

Suicide risk: Acute suicidal risk, e.g., active suicidal ideation with intent or plan, recent attempt, or clinical judgment requiring urgent intervention.

Interacting medications: Current use of serotonergic antidepressants (SSRI/SNRI/MAOI, clomipramine) or other pro-serotonergic agents (e.g., triptans, linezolid, lithium, tramadol, dextromethorphan) that cannot be discontinued per protocol-defined washout.

Other psychotropics: Unstable doses of antipsychotics, mood stabilizers, or long-acting benzodiazepines within the last 2 weeks; need for medications that would compromise blinding on the dosing day.

Psychotherapy changes: Initiation or major change in psychotherapy within 2 weeks prior to baseline (to preserve clinical stability).

Medical conditions: Clinically significant or unstable medical illness (cardiovascular, neurological, hepatic, renal), prolonged QTc, known hypersensitivity/contraindication to fluoxetine or study materials.

Pregnancy or breastfeeding. Substance use: Current substance use disorder (excluding nicotine/caffeine) within the past 3 months; non-medical cannabis use that cannot meet the pre-dose abstinence window (e.g., ≥72 h).

Any condition that, in the investigator's opinion, would make participation unsafe or interfere with the assessments.

Washout note (to include in Procedures/Eligibility):

SSRIs/SNRIs: 7 days or ≥5 half-lives; prior fluoxetine: ≥6 weeks; MAOIs: ≥14 days before randomization/dosing. Participants must be willing and able to follow the washout schedule.