2026 ICD-10-CM Diagnosis Code T40.426AUnderdosing of tramadol, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T40

ICD-10-CM T40.426A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T40.426A is a billable ICD-10-CM diagnosis code for underdosing of tramadol, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). The code is not accepted as a principal diagnosis by the Medicare Code Editor. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.

Code Identity

ICD-10-CM Code
T40.426A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of tramadol, initial encounter
Short Description
Underdosing of tramadol, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T40.426
Underdosing of tramadol

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT36-T50Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances
CategoryT40Poisoning by, adverse effect of and underdosing of narcotics and psychodysleptics [hallucinogens]
This CodeT40.426AUnderdosing of tramadol, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T40.426A.

Unacceptable principal diagnosis
There are selected codes that describe a circumstance which influences an individual's health status but not a current illness or injury, or codes that are not specific manifestations but may be due to an underlying cause. These codes are considered unacceptable as a principal diagnosis.

Coding GuidelinesGuidance

Underdosing refers to taking less of a medication than is prescribed by a provider or a manufacturer's instruction. Codes for underdosing should never be assigned as principal or first-listed codes. If a patient has a relapse or exacerbation of the medical condition for which the drug is prescribed because of the reduction in dose, then the medical condition itself should be coded.

The appropriate 7th character is to be added to each code from block Poisoning by, adverse effect of and underdosing of narcotics and psychodysleptics [hallucinogens] (T40). Use the following options for the applicable episode of care:

  • A - initial encounter
  • D - subsequent encounter
  • S - sequela

Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.

Clinical ClassificationClinical

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

CCSR INJ029
Underdosing of drugs and medicaments, initial encounter
Default principal diagnosis: inpatient No · outpatient No

Clinical InformationClinical

  • Tramadol

    a narcotic analgesic proposed for severe pain. it may be habituating.

Table of Drugs and ChemicalsClinical

Substances in the Table of Drugs and Chemicals that reference this code family. Always confirm in the Tabular List before coding.

SubstanceAccidentalSelf-harmAssaultUndeter­minedAdverse
Effect		Under­dosing
TramadolT40.421T40.422T40.423T40.424T40.425T40.426

Code History & ChangesHistory

Replacement T40.426A replaces the following previously assigned code(s):

  • T40.4X6A - Underdosing of other synthetic narcotics, initial encounter
FY 2021AddedAdded to the ICD-10-CM code setEffective October 1, 2020.
FY 2022–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T40.426AOverview

Is T40.426A (Underdosing of tramadol) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of tramadol, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T40.426A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of tramadol, such as an emergency visit or first evaluation.

Can T40.426A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of tramadol, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.