The study will enroll children and adolescents presenting with SI in the paediatric ED.  Inclusion Criteria:  10 to 17 years of age Respond "yes" to either question 1 (Passive Ideation) or 2 (Active Ideation) on the C-SSRS Respond "yes" to either questions 3 (Method), 4 (Intent), or 5 (Plan) on the C-SSRS Deemed acceptable and appropriate for admission to the on-site Child and Adolescent Psychiatry Emergency (CAPE) unit by a pediatric psychiatrist Normal vital signs for age and a normal neurological exam (no focal deficits or abnormalities), as per attending clinician  Exclusion Criteria:  History of benzodiazepine or ketamine use in the past 5 years Previous diagnosis of schizophrenia or active psychosis as per the treating physician Clinically unstable, requires resuscitation or admission to a medical ward for stabilizing therapy (i.e., intensive care unit.) Intoxicated or delirious Suspected or confirmed pregnancy Taking medications that may contraindicate the use of ketamine Known allergy or sensitivity to ketamine or ketamine-like compounds Neuro-cognitive impairment that precludes informed consent, assent, or ability to self-report pain and satisfaction Inability to understand spoken and/or written English without the use of an interpreter Previous enrollment in this study