Ketamine for OUD and Suicidal Ideation in the ED

Inclusion/Exclusion Criteria: To be eligible, individuals must be/have: * English speaking adults aged 18 and above * Diagnosed with DSM5 opioid use disorder, moderate or severe, or clinically suspected opioid use disorder * Endorsing suicidal ideation sufficiently severe to meet criteria for referral to an inpatient psychiatric facility or a crisis stabilization unit * Any prior history of an opioid overdose * Medically cleared Individuals with any of the following will be excluded: * Any psychotic disorder or active homicidally * Inability to perform consent due to impaired mental status * Current substance intoxication or current (or within the past 1-month) mania, hypomania, mixed-episode, or psychosis * Systolic blood pressure persistently elevated above 160mmHg, diastolic blood pressure greater than 100 mmHg, or heart rate \>100bmp, in the ED * Clinical Opioid Withdrawal Scale (COWS) score of 12 or greater * History of hypersensitivity to ketamine, or experience of emergence reaction * History of hypersensitivity to ondansetron or concurrently using apomorphine * History of any illicit or recreational use of ketamine in the past 12 months * Receipt of ketamine treatment for depression in the past 3 months * History of DSM5 hallucinogen use disorder, intracranial mass or bleed, porphyria, thyrotoxicosis, seizure disorder other than from alcohol withdrawal, liver cirrhosis, or sleep apnea * Cardiac or EKG abnormalities based on history, physical examination or the baseline EKG * History within 6 months of thoracic surgery, lung cancer, head trauma, stroke, or myocardial infarction * Liver dysfunction with LFTs \>3x upper normal limit * Current use of medications with known drug-drug interactions with ketamine (i.e., St. John's Wort, theophylline, opioid analgesics other than buprenorphine and methadone for the treatment OUD, Central Nervous System (CNS) depressants other than benzodiazepines or phenobarbital) * Pregnant * Patients who are breastfeeding * ASA class 3 or greater or documented history of difficult airway in HER * Active exacerbation of COPD or asthma * Currently participating or anticipated to participate in a concurrent investigational clinical trial * High risk for adverse emotional or behavioral reaction based on the opinion of the study investigators, including evidence of a personality disorder