2027 ICD-10-CM Diagnosis Code G90.81Serotonin syndrome

ICD-10-CM Codes›G00–G99›G89-G99›G90

ICD-10-CM G90.81
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

G90.81 is a billable ICD-10-CM diagnosis code for serotonin syndrome. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 73 through 74. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (neither hereditary nor degenerative).

G90.81 does not risk-adjust for Medicare Advantage under the CMS-HCC V28 model. It still risk-adjusts in the ESRD (V21) category 75 for payment year 2027.

Code Identity

ICD-10-CM Code
G90.81
Billable Status
Yes — Valid for Submission
Code Describes
Serotonin syndrome
Short Description
Serotonin syndrome
Same as the full description in the CMS dataset.
Parent Code
G90.8
Other disorders of autonomic nervous system

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG89-G99Other disorders of the nervous system
CategoryG90Disorders of autonomic nervous system
This CodeG90.81Serotonin syndrome

Medicare Risk Adjustment (HCC)Billing

G90.81 does not risk-adjust for Medicare Advantage under the CMS-HCC V28 model, but it maps to payment categories in the other CMS models shown below.

CMS-HCC V28 (Medicare Advantage Payment Model)
Not mapped
no payment category in the model that pays 100% of MA risk scores
Prior Model (CMS-HCC V24)
Not mapped under V24
this diagnosis newly risk-adjusts under the V28 model
Other CMS Models
ESRD (V21): HCC 75
ESRD V21 weights: 0.059 dialysis, 0.284–0.323 functioning graft
Part D (RxHCC)
Not mapped
G90.81 does not risk-adjust in the RxHCC prescription drug model

Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Serotonin syndrome

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G90.81: its own notes plus those printed at G90 and Chapter 6. A note printed at a category, block or chapter applies to every code under it.

Applicable To

Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.

  • Serotonin toxicity

Excludes1

Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.

From G90 Disorders of autonomic nervous system applies to 21 codes
  • dysfunction of the autonomic nervous system due to alcohol (G31.2)

Excludes2

Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.

From Chapter 6 (G00-G99) Diseases of the nervous system applies to 700 codes
  • 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)

Code First

The underlying condition named in the note is sequenced before 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 a second code, after this one, to fully describe the condition.

  • 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-)

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS020
Other nervous system disorders (neither hereditary nor degenerative)
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Serotonin Syndrome

    an adverse drug interaction characterized by altered mental status, autonomic dysfunction, and neuromuscular abnormalities. it is most frequently caused by use of both serotonin reuptake inhibitors and monoamine oxidase inhibitors, leading to excess serotonin availability in the cns at the serotonin 1a receptor.

Code History & ChangesHistory

Replacement G90.81 replaces the following previously assigned code(s):

  • G90.8 - Other disorders of autonomic nervous system
FY 2025AddedAdded to the ICD-10-CM code setEffective October 1, 2024.
FY 2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About G90.81Overview

What is the ICD-10 code for serotonin syndrome?

The ICD-10-CM code for serotonin syndrome is G90.81 (sometimes written as G9081). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is G90.81 (Other disorders of autonomic nervous system) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report serotonin syndrome on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does G90.81 group to?

When serotonin syndrome is the principal diagnosis on an inpatient stay, it groups to MS-DRG 74, 73, with relative weights from 1.0172 to 1.5773 depending on complications. Higher weights mean higher Medicare reimbursement.

Can G90.81 be reported with G31.2?

Not as a rule. The Excludes1 note at G90, which applies to G90.81, lists "dysfunction of the autonomic nervous system due to alcohol (G31.2)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).

Does G90.81 need an additional code?

When the condition applies, yes. The Use Additional Code note reads: 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:; and 6 more listed in the Instructional Notes section. The additional code is reported after G90.81.

Does G90.81 risk-adjust for Medicare Advantage payment?

Not for Medicare Advantage: G90.81 maps to no category in the CMS-HCC V28 model. It still risk-adjusts in the ESRD (V21) category 75 (Polyneuropathy).