Inclusion Criteria:
* Age between 25 and 55 years
* BMI between 19 and 35 kg/m2
* Voluntary, written, informed consent
* Physically healthy by medical history, physical, neurological, ECG, and laboratory examinations
* DSM-5 criteria for Opioid Use Disorder
* Documented evidence (by urine toxicology) of opioid use (upon screening)
* Inpatient verified \> 1 week of abstinence from illicit opioids
* For females, a negative serum pregnancy (beta-HCG) test
* Participants are required to commit to employing dual contraceptive methods throughout the study and to abstain from sperm or egg donation during the study period and for 28 days following the final drug dose for ova, and for 90 days following the final drug dose for sperm. Dual contraceptive methods encompass the use of a barrier contraceptive, such as condoms, coupled with another effective method capable of preventing pregnancy, such as oral or parenteral contraceptives, intrauterine devices, spermicide, and the like.
Exclusion Criteria:
* DSM-5 criteria for other substance use disorders (e.g., alcohol, cocaine, sedative hypnotics), except for nicotine (concurrent alcohol or drug use is allowed if it does not meet criteria for a substance use disorder and does not take place during inpatient stay)
* A primary DSM-5 Axis I diagnosis of schizophrenia, schizoaffective disorder, bipolar disorder, or major depression, as determined by psychiatric history (Mini International Neuropsychiatric Interview, MINI) (Sheehan et al., 1998), or another disorder that may interfere with the study's primary outcomes in the view of PI
* Immediate (first-degree relative) family history of formally diagnosed schizophrenia or other psychotic disorders (e.g., delusional disorder, schizoaffective disorder), or bipolar I/II disorder
* Individuals with a history or evidence of psychosis, including substance and nonsubstance related, as evaluated in assessments (MINI and Brief Psychiatric Rating Scale \[BPRS\]) before psilocybin administration
* History of Hallucinogen Use Disorder or Hallucinogen Persisting Perceptive Disorder
* A history of significant and/or uncontrolled medical or neurological illness
* Hypertension at screening defined as: systolic blood pressure \> 140 mmHg or diastolic blood pressure \> 90 mmHg
* Heart rate outside the range of 60 to 100 beats per minute
* History of cardiovascular disease, including but not limited to clinically significant coronary artery disease, cardiac hypertrophy, cardiac ischemia, congestive heart failure, myocardial infarction, angina pectoris, coronary artery bypass graft or artificial heart valve, stroke, transient ischemic attack, or any clinically significant arrhythmia
* Any clinically significant abnormal electrocardiogram (ECG) finding, such as findings suggestive of ischemia or infarct, complete bundle branch block, atrial fibrillation or other symptomatic arrhythmia, or predominantly non-sinus rhythm, at screening
* Resting QT interval with Fridericia's correction (QTcF) ≥ 450 msec at Screening, or inability to determine QTcF interval
* Presence of risk factors for torsades de pointes, including: long QT syndrome, uncontrolled hypokalemia or hypomagnesemia, history of cardiac failure, history of clinically significant/symptomatic bradycardia, family history of idiopathic sudden death or congenital long QT syndrome, or concomitant use of a torsadogenic medication
* Current use of psychotropic and/or potentially psychoactive prescription medications considered to the investigators are likely to interfere clinically with human subject's safety (i.e., contraindicated drug-drug interactions with psilocybin) or scientifically (i.e., likely to influence or alter outcomes of the study)
* Current use of medications with serotonergic activity, as participants on these medications are at risk of serotonin syndrome or drug-drug interactions
* Medical contraindications to MRI procedures (e.g., ferromagnetic implants/foreign bodies, claustrophobia, etc.)
* Arterial Line Exclusion: Blood donation within eight weeks of the start of the study
* Arterial Line Exclusion: History of a bleeding disorder or are currently taking anticoagulants (such as Coumadin, Heparin, Pradaxa, Xarelto)
* Participation in other research studies involving ionizing radiation within one year of the PET scans that would cause the subject to exceed the yearly dose limits followed by the Yale PET Center (21CFR361.1)
* Moderate to severe hepatic impairment (Child-Pugh B and C class)
* Use of enzyme inhibitors (UGT1A9, UGT1A10, MAO, and aldehyde or alcohol dehydrogenase).
