Alphabetic Index FY2027

G90.81

Serotonin syndrome

FY2027Chapter 6: Diseases of the nervous system/G89–G99 Other disorders of the nervous system

BillableComplete code, valid to report
Care settingsHome Health Primary · Neuro Rehabilitation

G90.81 (also written G9081) is a billable ICD-10-CM diagnosis code for serotonin syndrome. It is part of the FY2027 code set, effective October 1, 2026. It is one of the 21 codes in category G90 (Disorders of autonomic nervous system). The Alphabetic Index lists it under 1 entry. Its coding notes include a Code first instruction, a Use additional code instruction, 1 Excludes1 entry and 8 Excludes2 entries.

Code first

If the underlying condition listed here is present and known, sequence it first.

This code
  • poisoning due to drug or toxin, such as:
    • linezolid (T36.8X- with sixth character 1-4)
    • monoamine oxidase inhibitors (T43.1X with sixth character 1-4)
    • selective serotonin and norepinephrine reuptake inhibitors [SSNRI] (T43.21 with sixth character 1-4)
    • selective serotonin reuptake inhibitors [SSRI] (T43.22 with sixth character 1-4)

Use additional code

Add the listed code as a secondary code, if known.

This code
  • code for adverse effect, if applicable, to identify drug, such as:
    • linezolid (T36.8X5)
    • monoamine oxidase inhibitors (T43.1X5)
    • selective serotonin and norepinephrine reuptake inhibitors [SSNRI] (T43.215)
    • selective serotonin reuptake inhibitors [SSRI] (T43.225)
  • code, if applicable, to identify:
    • disseminated intravascular coagulation (D65)
    • hypertensive crisis (I16.-)
    • metabolic acidosis (E87.2-)
    • shock, not elsewhere classified (R57.-)
    • toxic encephalopathy (G92.-)
    • ventricular tachycardia (I47.2-)

Excludes1

Not coded here. The listed conditions generally should not be reported together.

Inherited from G90 Disorders of autonomic nervous system
  • dysfunction of the autonomic nervous system due to alcohol (G31.2)

Excludes2

Not included here. Both codes may be reported if the patient has both conditions.

Also from chapter/block level (8 notes)
Inherited from Chapter 6 Diseases of the nervous system
  • certain conditions originating in the perinatal period (P04-P96)
  • certain infectious and parasitic diseases (A00-B99)
  • complications of pregnancy, childbirth and the puerperium (O00-O9A)
  • congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
  • endocrine, nutritional and metabolic diseases (E00-E88)
  • injury, poisoning and certain other consequences of external causes (S00-T88)
  • neoplasms (C00-D49)
  • symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)

Care settings

How CMS programs treat this code. These are CMS claim edits and list entries, not coverage or eligibility rules.

Home Health Primary

Accepted as the primary (principal) diagnosis on a home health claim. The Home Health Grouper assigns it to the Neuro Rehabilitation clinical group.

Source: CMS Home Health Grouper Software v07.2.26, effective October 1, 2026

Inclusion terms

Conditions classified to this code.

This code
  • Serotonin toxicity

Alphabetic Index terms

Terms the official Alphabetic Index lists for this code. 1 entry