Inclusion Criteria:

1. 18-69 years old
2. Have been on a stable psychiatric medication regimen for at least four weeks prior to study participation.
3. Formal diagnosis of major depressive disorder or post-traumatic stress disorder by a mental health professional.
4. Must be under the care of a medical provider (ex: primary care, therapist, psychiatrist, psychiatric nurse practitioner) for mental health treatment who can provide contact information and written confirmation that they will be willing and able to care for participant during their entire involvement in the study.

Exclusion Criteria:

1. A prior history of other central nervous system disease or any history of seizures;
2. history of psychotic disorders (e.g., schizophrenia, schizoaffective disorder, bipolar disorder type I);
3. history of current or recent (within one year) substance/alcohol use disorder, with the exception of tobacco use disorder;
4. meet criteria for Very High risk of suicide, or require inpatient hospital-level care for psychiatric reasons at time of consent, to reduce exacerbation of risk of harm to self during study;
5. presence of any implanted metal or electrical device (e.g. pacemaker);
6. recent medical hospitalization (within three weeks);
7. any condition that would prevent the participant from completing the protocol, such as significant agitation;
8. appointment of a legal representative or treatment guardian;
9. any ongoing litigation related to a health condition;
10. any other contraindication to exposure to strong magnetic fields or MRI, such as severe claustrophobia;
11. pregnancy or lactation;
12. a family history of schizophrenia or schizoaffective disorder (first or second degree relatives), or bipolar disorder type 1 (first degree relatives), to reduce risk of exacerbation of an undiagnosed psychotic condition;
13. other medical conditions that would preclude safe participation in the trial (e.g., decompensated heart failure);
14. starting or planning to start psychotherapy or changing the frequency or intensity of existing psychotherapy during the trial (current psychotherapy can be continued provided the frequency and intensity has been stable for ≥2 months prior to screening);
15. membership in a vulnerable population (minors, prisoners);
16. any contraindication for blood draws.

Arm 1:

Inclusion:

1. PRE-EMPT Risk Level Low or Intermediate (recent suicidal ideation, but no intent/plan; C-SSRS level low; mild-to-moderate depression (QIDS score 6-15) or PTSD (PCL-5 score 20-32))
2. Right-handed.

Arm 2:

Inclusion:

1) PRE-EMPT Risk Level Intermediate or High (recent suicidal ideation, but no intent or plan; C-SSRS level low to moderate; moderate to severe depression (QIDS score >16) or PTSD (PCL-5 score greater than or equal to 33))

Arm 3:

Inclusion:

1) PRE-EMPT Risk Level Intermediate or High (recent suicidal ideation, but no intent or plan; C-SSRS level low to moderate; moderate to severe depression (QIDS score ≥16) or PTSD (PCL-5 score greater than or equal to 33))

Exclusions:

1. Other additional medical conditions that would preclude safe participation in this arm of the trial:

   1. significantly impaired liver function,
   2. severe coronary artery disease,
   3. heart failure,
   4. uncontrolled hypertension (above 140/90 mmHg upon screening) ,

   i. If blood pressure is consistently elevated > 140/90 mmHg across 3 attempts, subjects may be referred to their primary care provider for management of hypertension. Upon management of blood pressure, subjects will have an opportunity to return once within the 30 day screening window to make 3 additional attempts at a blood pressure reading ≤ 140/90 mmHg. Subjects will be considered eligible upon registering 1 blood pressure reading ≤ 140/90 mmHg during the screening period. Blood pressure will be reassessed on Day 0 prior to dosing, and must be less than or equal to 140 systolic, 90 diastolic, with resting pulse ≤ 100 (ascertained within 20 minutes of IP administration in order for the participant to receive study medication.) e. history of cerebrovascular accident, f. severe obesity (BMI ≥ 35), g. untreated asthma, h. untreated hyperthyroidism, i. narrow-angle glaucoma, j. stenosing peptic ulcer, k. pyloroduodenal obstruction, l. symptomatic prostatichypertrophy, or m. bladder-neck obstruction, n. history of valvular heart disease or any heart condition that affects the valves

2. serious ECG abnormalities (e.g., evidence of ischemia, myocardial infarction, QT interval corrected for heart rate [QTc] prolongation (QTc > 0.45 seconds), arrhythmia, or conduction abnormalities that increase the risk of arrhythmia);

3. allergy or hypersensitivity to psilocybin or chocolate