2026 ICD-10-CM Diagnosis Code T40.3X6AUnderdosing of methadone, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T40

ICD-10-CM T40.3X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T40.3X6A is a billable ICD-10-CM diagnosis code for underdosing of methadone, 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
T40.3X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of methadone, initial encounter
Short Description
Underdosing of methadone, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T40.3X6
Underdosing of methadone

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 CodeT40.3X6AUnderdosing of methadone, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T40.3X6A.

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

  • Methadone

    a synthetic opioid that is used as the hydrochloride. it is an opioid analgesic that is primarily a mu-opioid agonist. it has actions and uses similar to those of morphine. (from martindale, the extra pharmacopoeia, 30th ed, p1082-3)

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
AmidoneT40.3X1T40.3X2T40.3X3T40.3X4T40.3X5T40.3X6
DolophineT40.3X1T40.3X2T40.3X3T40.3X4T40.3X5T40.3X6
Levo-iso-methadoneT40.3X1T40.3X2T40.3X3T40.3X4T40.3X5T40.3X6
MethadoneT40.3X1T40.3X2T40.3X3T40.3X4T40.3X5T40.3X6
PhyseptoneT40.3X1T40.3X2T40.3X3T40.3X4T40.3X5T40.3X6

Patient EducationClinical

Medication Errors

Medicines treat infectious diseases, prevent problems from chronic diseases, and ease pain. But medicines can also cause harmful reactions if not used correctly. Errors can happen in the hospital, at the health care provider's office, at the pharmacy, or at home. You can help prevent errors by:

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert T40.3X6A to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
No Map There is no ICD-9 equivalent for this code.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T40.3X6AOverview

Is T40.3X6A (Underdosing of methadone) a billable code?

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

What does the 7th character A in T40.3X6A mean?

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

Can T40.3X6A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of methadone, 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.