The purpose of this study is to examine the effect of a low-dose ketamine infusion on depression symptoms within the Emergency Department (ED) visit, and healthcare utilization after leaving the ED, when administered in the ED for depression or suicidal ideation.

Patients will be identified based on either a chief complaint of suicidal ideation, suicide attempt, or severe depression, or if the patient indicates during intake assessment that they have thoughts of harming or killing their self. Identified patients will be assessed by the ED provider for inclusion and exclusion criteria. If the patient is a candidate for the trial and gives their informed consent for enrollment, they will be randomized to receive either ketamine or placebo by a computer program that maintains blinding. The patient will be assessed by psychiatry for disposition and treatment plan prior to receiving the study drug, and will be reassessed four hours after the infusion. Thirty days after the patient leaves the hospital (either leaves from the ED or is discharged from inpatient admission), they will be contacted to follow up on their healthcare utilization after discharge.

Inclusion Criteria:

• Adults 18 years of age and older
• Presenting to the ED with a chief complaint of severe depression or suicidality, or presenting to the ED with any other chief complaint but answering "yes" to "Thoughts of harming/killing yourself?" during intake/triage

Exclusion Criteria:

• Acute mania or psychosis
• Enrollment in trial during a prior emergency department visit
• History of ketamine abuse or dependence
• Known hypersensitivity to ketamine
• Acute intoxication with any drug of abuse (including alcohol)
• Pregnancy or lactation
• Any condition that would place the patient at serious risk of harm from an increase in blood pressure (e.g. history of intracerebral hemorrhage, aneurysmal vascular disease, or arteriovenous malformation)
• Assessing provider does not want to enroll patient for any other reason, based on their clinical judgement