ICD-10 Code G90.81 – Serotonin Syndrome (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code G90.81 – Serotonin syndrome
What it is
Serotonin syndrome is a toxic reaction from too much serotonergic activity in the body. It usually follows use, overdose, or interaction of medicines that increase serotonin.
Clinical signs
Typical findings include mental status changes, autonomic instability, and neuromuscular abnormalities. Clinical features vary; refer to documentation.
When to use this code
Use this code when the record clearly documents serotonin syndrome, serotonin toxicity, or a similar confirmed diagnosis. It may apply after therapeutic use, medication error, overdose, or drug interaction if the provider identifies this condition.
Do not code it from nonspecific agitation, fever, or tremor alone. Check documentation for the clinician’s final assessment and any stated cause.
Do not use for
Do not use this code for general adverse drug effects, poisoning without serotonin syndrome, or isolated medication side effects. If serotonin syndrome is not documented, check documentation.
Coding tip
Capture the provider’s exact diagnosis wording and any documented drug cause, because the underlying medication event may need separate coding.