Serotonin syndrome is a potentially life-threatening complication of psychopharmacologic drug therapy. The syndrome is produced most often by the concurrent use of two or more drugs that increase brainstem serotonin activity and is often unrecognized because of the varied and nonspecific nature of its symptomatology. Serotonin syndrome is characterized by alterations in cognition, behavior, autonomic nervous system function and neuromuscular activity. Patients with serotonin syndrome usually respond to discontinuation of drug therapy and supportive care alone, but they may require treatment with a specific antiserotonergic drug such as cyproheptadine, methysergide and/or propranolol.