Time Course of Serum Dextromethorphan Concentrations in a Case of Serotonin Syndrome After Massive Overdose

BACKGROUND Dextromethorphan (DXM) inhibits serotonin reuptake when taken in large doses, potentially causing serotonin syndrome, which is likely life-threatening. Despite a few reports of serotonin syndrome after DXM overdose, no case report has provided a detailed analysis of serial blood DXM levels alongside the clinical course. Here, we present a case with serial measurements of blood DXM levels taken over time. CASE REPORT A 24-year-old man with bipolar disorder and attention-deficit/hyperactivity disorder, treated with trazodone and methylphenidate, ingested 300 mg of DXM (4.6 mg/kg). After 10 hours, he exhibited agitation, confusion, and impaired consciousness. On admission, his DXM blood level was 398 ng/mL; trazodone and methylphenidate levels were both <1 ng/mL. He developed serotonin syndrome with spontaneous clonus and seizures, requiring diazepam, sedation, intubation, and ventilation. By hospital day 3, the DXM level decreased to 26 ng/mL; by day 5, it was undetectable. Despite clinical stabilization and extubation by day 4, hallucinations and delusions persisted. After receiving a diagnosis of acute and transient psychotic disorder on day 9, he was transferred to the psychiatry department. With oral therapy and electroconvulsive therapy, he improved by day 33. The final diagnosis was substance/medication-induced psychotic disorder. CONCLUSIONS Our case report provides a detailed timeline of blood DXM concentrations along with the clinical course in a patient with serotonin syndrome after an acute overdose. The serial measurements support the importance of symptom-focused monitoring to guide treatment decisions.