Serotonin syndrome is a rare but potentially life-threatening condition caused by excess serotonin in the central and peripheral nervous systems. Patients with serotonin syndrome present with a range of mild to severe autonomic, neuromuscular and mental state signs and symptoms. A variety of drugs affect the serotonin pathways by modifying serotonin release and reuptake mechanisms, or reducing metabolism. There are also several genetic polymorphisms and clinical risk factors that affect the development and course of serotonin syndrome. This article describes the pathophysiology of serotonin syndrome and discusses diagnosis and treatment with reference to a case study of a patient who attended an emergency department (ED) with signs and symptoms of the condition following an increase in antidepressant medicines. The article aims to increase clinicians’ awareness of serotonin syndrome to improve identification and treatment of patients who present to EDs with the condition.