Uncontrolled opioid withdrawal and pain often drive inpatients with opioid use disorder to leave hospital against medical advice, resulting in suboptimal medical and addiction treatment. When oral opioid agonist treatments such as methadone and buprenorphine/naloxone fail for management of craving and withdrawal, injectable opioid agonist treatment may serve to retain patients in care and link them to addiction services. We describe the case of a 47-year-old man with a severe, active opioid use disorder and daily use of illicitly manufactured fentanyl, who was re-admitted to hospital for post-operative management after leaving against medical advice due to uncontrolled opioid withdrawal. Intravenous hydromorphone was used to retain him in care, allowing for completion of his antibiotics and enrolment in ongoing community injectable opioid agonist treatment. [McAdam M, Brar R, Young S. Initiation of injectable opioid agonist treatment in hospital: A case report. Drug Alcohol Rev 2020;39:138-141]