Serotonin syndrome

Serotonin syndrome is a rare but potentially life-threatening condition caused by an excess of serotonin.  It usually affects people who are taking two drugs which increase serotonin levels and usually develops within a few days of a change in medication or increase in drug dose.  Hence a thorough drug history is essential for any patient who may be taking neuropsychiatric drugs.  Always include prescription, OTC, complementary therapies and illegal drugs in your history.

Some of the drugs which can precipitate serotonin syndrome are:

  • Antidepressants
  • Tramadol
  • Fentanyl
  • Dextromethorphan
  • Ondansetron
  • Cocaine
  • MDMA
  • Amphetamines
  • LSD

Serotonin syndrome symptoms:

  • tachycardia
  • hypertension
  • hyperthermia
  • confusion
  • agitation
  • sweating
  • mydriasis
  • rhabdomyolysis
  • hyperreflexia
  • tremor
  • clonus

Neuroleptic malignant syndrome (NMS) shares many symptoms with serotonin syndrome, but doesn’t normally cause hyperreflexia. NMS usually affects people taking antipsychotic medications and can occur even when the patient has been taking the same drug(s) for some time with no previous adverse effects.

Differential diagnoses:

  • Hyperthyroidism
  • Malignant hyperthermia
  • Encephalitis
  • Delirium tremens
  • Drugs e.g. anticholinergics, amphetamines
  • Meningitis
  • Neuroleptic malignant syndrome


  1. Patient Access: Serotonin Syndrome
  2. Patient Access: Neuroleptic Malignant Syndrome
  3. Boyer, ED. 2018. Serotonin syndrome (serotonin toxicity)
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