2026 ICD-10-CM Diagnosis Code T42.0X6AUnderdosing of hydantoin derivatives, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T42

ICD-10-CM T42.0X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T42.0X6A is a billable ICD-10-CM diagnosis code for underdosing of hydantoin 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
T42.0X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of hydantoin derivatives, initial encounter
Short Description
Underdosing of hydantoin derivatives, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T42.0X6
Underdosing of hydantoin 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
CategoryT42Poisoning by, adverse effect of and underdosing of antiepileptic, sedative- hypnotic and antiparkinsonism drugs
This CodeT42.0X6AUnderdosing of hydantoin derivatives, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T42.0X6A.

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 antiepileptic, sedative- hypnotic and antiparkinsonism drugs (T42). 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

  • Mephenytoin

    an anticonvulsant effective in tonic-clonic epilepsy (epilepsy, tonic-clonic). it may cause blood dyscrasias.

  • Phenytoin

    an anticonvulsant that is used to treat a wide variety of seizures. it is also an anti-arrhythmic and a muscle relaxant. the mechanism of therapeutic action is not clear, although several cellular actions have been described including effects on ion channels, active transport, and general membrane stabilization. the mechanism of its muscle relaxant effect appears to involve a reduction in the sensitivity of muscle spindles to stretch. phenytoin has been proposed for several other therapeutic uses, but its use has been limited by its many adverse effects and interactions with other drugs.

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
AlbutoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
DilantinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
DiphenylhydantoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
EpanutinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
EthotoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Hydantoin derivative NECT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MephenytoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Mephenytoin::with phenobarbitalT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MetetoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MethetoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
MethoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
PhenantoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
PhenytoinT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6
Phenytoin::with PhenobarbitalT42.0X1T42.0X2T42.0X3T42.0X4T42.0X5T42.0X6

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 T42.0X6A 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 T42.0X6AOverview

Is T42.0X6A (Underdosing of hydantoin derivatives) a billable code?

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

What does the 7th character A in T42.0X6A mean?

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

Can T42.0X6A be a principal diagnosis?

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