2026 ICD-10-CM Diagnosis Code T39.1X6AUnderdosing of 4-Aminophenol derivatives, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T39

ICD-10-CM T39.1X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T39.1X6A is a billable ICD-10-CM diagnosis code for underdosing of 4-Aminophenol derivatives, 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
T39.1X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of 4-Aminophenol derivatives, initial encounter
Short Description
Underdosing of 4-Aminophenol derivatives, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T39.1X6
Underdosing of 4-Aminophenol derivatives

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
CategoryT39Poisoning by, adverse effect of and underdosing of nonopioid analgesics, antipyretics and antirheumatics
This CodeT39.1X6AUnderdosing of 4-Aminophenol derivatives, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T39.1X6A.

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 nonopioid analgesics, antipyretics and antirheumatics (T39). 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

  • Acetaminophen

    analgesic antipyretic derivative of acetanilide. it has weak anti-inflammatory properties and is used as a common analgesic, but may cause liver, blood cell, and kidney damage.

  • Arylsulfotransferase

    a sulfotransferase that catalyzes the sulfation of a phenol in the presence of 3'-phosphoadenylylsulfate as sulfate donor to yield an aryl sulfate and adenosine 3',5'-bisphosphate. a number of aromatic compounds can act as acceptors; however, organic hydroxylamines are not substrates. sulfate conjugation by this enzyme is a major pathway for the biotransformation of phenolic and catechol drugs as well as neurotransmitters. ec 2.8.2.1.

  • Cytochrome P-450 CYP1A2

    a cytochrome p450 enzyme subtype that has specificity for relatively planar heteroaromatic small molecules, such as caffeine and acetaminophen.

  • Phenacetin

    a phenylacetamide that was formerly used in analgesics but nephropathy and methemoglobinemia led to its withdrawal from the market. (from smith and reynard, textbook of pharmacology,1991, p431)

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
P-AcetamidophenolT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
AcetaminophenT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
AcetaminosalolT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
AcetanilideT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
AcetophenetedinT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
4-Aminophenol derivativesT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
Bromo-seltzerT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
ExalginT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
PanadolT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
Para-acetamidophenolT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
Para-aminophenol derivativesT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
ParacetamolT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6
PhenacetinT39.1X1T39.1X2T39.1X3T39.1X4T39.1X5T39.1X6

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 T39.1X6A 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 T39.1X6AOverview

Is T39.1X6A (Underdosing of 4-Aminophenol derivatives) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of 4-Aminophenol derivatives, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T39.1X6A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of 4-Aminophenol derivatives, such as an emergency visit or first evaluation.

Can T39.1X6A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of 4-Aminophenol derivatives, 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.