Serotonin syndrome

Page most recently updated 7 September 2026

This page focuses on the reported psychiatric associations, which may be the initial presenting manifestation, and on other clinical features that may assist the psychiatrist or primary care physician in identifying the underlying diagnosis, considered through the lenses of the diagnostic lenses framework.

Serotonin syndrome is a potentially life-threatening toxicologic disorder caused by excessive serotonergic activity, typically occurring after the initiation, dose escalation, combination, or overdose of serotonergic medications or substances. A distinctive presentation may include the combination of acute neuropsychiatric change, neuromuscular hyperactivity, autonomic instability, and a recent serotonergic medication change or exposure.

It can present with psychiatric features such as agitation, anxiety, restlessness, irritability, emotional lability, insomnia, pressured speech, and behavioural change. In more severe cases, confusion, delirium, and perceptual disturbances may occur. Rarely, psychotic symptoms have been reported in the absence of overt delirium, although these are not characteristic features of serotonin syndrome. The acute onset and close temporal relationship to a serotonergic medication change or exposure are important diagnostic clues.

Other characteristic features include tremor, hyperreflexia, myoclonus, hypertonia, and particularly spontaneous, inducible, or ocular clonus. Neuromuscular findings are often more prominent in the lower limbs. Severe toxicity may progress rapidly to marked hyperthermia, generalized rigidity, seizures, rhabdomyolysis, metabolic acidosis, and multiorgan dysfunction.

From a multisystemic lens perspective, clinical features in other body systems include (but are not limited to):

  • Cardiovascular: Tachycardia, hypertension or hypotension, and other manifestations of autonomic instability.

  • Gastrointestinal: Increased bowel activity, nausea, vomiting, and diarrhea.

Early diagnosis is important because serotonin syndrome can progress rapidly from mild toxicity to severe hyperthermia, rigidity, rhabdomyolysis, metabolic complications, multiorgan dysfunction, and other life-threatening complications. Prompt recognition allows serotonergic agents to be discontinued and supportive treatment initiated, with more severe cases requiring urgent medical management.

Selected references and further reading — Multisystemic lens

Cardinal RN, Bullmore ET. The diagnosis of psychosis. Cambridge: Cambridge University Press; 2011.

Sachdev PS, Keshavan MS, editors. Secondary schizophrenia. Cambridge: Cambridge University Press; 2010.

Levenson JL, editor. The American Psychiatric Association Publishing textbook of psychosomatic medicine and consultation-liaison psychiatry. 3rd ed. Washington (DC): American Psychiatric Association Publishing; 2019.

Stern TA, Beach SR, Smith FA, Freudenreich O, Vranceau AM, Fava M, editors. Massachusetts General Hospital handbook of general hospital psychiatry. 8th ed. Philadelphia: Elsevier; 2025.

Arciniegas DB, Yudofsky SC, Hales RE, editors. The American Psychiatric Association Publishing textbook of neuropsychiatry and clinical neurosciences. 6th ed. Washington (DC): American Psychiatric Association Publishing; 2018.

Agrawal N, Faruqui R, Bodani M, editors. Oxford textbook of neuropsychiatry. Oxford: Oxford University Press; 2020.

Boland R, Verduin M, editors. Kaplan and Sadock's comprehensive textbook of psychiatry. 11th ed. Philadelphia: Wolters Kluwer; 2024.

Loscalzo J, Fauci AS, Kasper DL, Hauser SL, Longo DL, Jameson JL, editors. Harrison's principles of internal medicine. 22nd ed. New York: McGraw Hill; 2025.