Psilocybin FCR Early-Stage Breast Cancer

Inclusion Criteria:

* Aged ≥ 21
* Females
* Diagnosis of Stage 1 or 2 Estrogen/Progesterone Receptor-Positive (ER/PR +), Human Epidermal growth factor Receptor 2 negative (HER2-) breast cancer
* Having completed initial treatment (e.g. surgery, chemotherapy, radiotherapy) for breast cancer (ongoing adjuvant endocrine, and/or HER2 therapy allowed)
* At least 1 year after completing initial treatment
* Clinically significant Fear of Cancer Recurrence (FCR) defined as Fear of Cancer Recurrence Inventory (FCRI) \> 20 at Screening
* Have an identified support person
* Agree to be accompanied home (or to an otherwise safe destination) by the support person, or another responsible party, following dosing
* Participants of childbearing potential must agree to practice an effective means of birth control throughout the duration of the study.

Exclusion Criteria:

* Unstable medical conditions or serious abnormalities of complete blood count, chemistries, or ECG that in the opinion of the study physician would preclude safe participation in the trial.
* Risk for hypertensive crisis defined as Screening, Baseline, and Medication Session (prior to dosing) BP \>160/100, HR \>110 bpm
* Significant central nervous system (CNS) pathology.
* Primary psychotic or affective psychotic disorders.
* Family history of first-degree relative with psychotic or serious bipolar spectrum illnesses.
* Clinically significant suicidality or high risk of completed suicide.
* History of hallucinogen persisting perception disorder (HPPD)
* Pregnancy/lactation
* Concurrent Medications
* Psychotherapy
* Have any psychological or physical symptom, medication or other relevant finding prior to randomization, based on the clinical judgment of the PI or relevant clinical study staff that would make a participant unsuitable for the study.
* Be enrolled in another clinical trial assessing intervention(s) for anxiety, depression, and/or existential distress (e.g., pharmacologic or psychotherapeutic interventions)