2026 ICD-10-CM Diagnosis Code T42.0X5SAdverse effect of hydantoin derivatives, sequela
T42.0X5S is a billable ICD-10-CM diagnosis code for adverse effect of hydantoin derivatives, sequela. The 7th character S marks it as a sequela code, which reports a problem that remains after the original condition has resolved. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 922 through 923. The code is not accepted as a principal diagnosis by the Medicare Code Editor and exempt from POA reporting. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Poisoning/toxic effect/adverse effects/underdosing, sequela.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T42.0X5S.
Present on Admission (POA)Billing
T42.0X5S is exempt from POA reporting on inpatient claims to general acute care hospitals. Review other POA exempt codes.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Adverse reaction to hydantoin and/or hydantoin derivative
- Class I antiarrhythmic adverse reaction
- Drug-induced erythema multiforme
- Drug-induced lupus erythematosus
- Drug-induced lupus erythematosus caused by diphenylhydantoin
- EMPACT syndrome
- Gingival disease caused by drug
- Gingival hypertrophy caused by diphenylhydantoin
- Hypertrophy of gingiva
- Phenytoin adverse reaction
- Purple glove syndrome
Coding GuidelinesGuidance
When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug.
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.
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.
| Substance | Accidental | Self-harm | Assault | Undetermined | Adverse Effect | Underdosing |
|---|---|---|---|---|---|---|
| Albutoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Dilantin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Diphenylhydantoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Epanutin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Ethotoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Hydantoin derivative NEC | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Mephenytoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Mephenytoin::with phenobarbital | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Metetoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Methetoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Methoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Phenantoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Phenytoin | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
| Phenytoin::with Phenobarbital | T42.0X1 | T42.0X2 | T42.0X3 | T42.0X4 | T42.0X5 | T42.0X6 |
Patient EducationClinical
Drug Reactions
Most of the time, medicines make our lives better. They reduce aches and pains, fight infections, and control problems such as high blood pressure or diabetes. But medicines can also cause unwanted reactions, such as drug interactions, side effects, and allergies.
The full article covers:
- What is a drug interaction?
- What are side effects?
- What are drug allergies?
- How can I stay safe when taking medicines?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T42.0X5S to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T42.0X5SOverview
Is T42.0X5S (Adverse effect of hydantoin derivatives) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of hydantoin derivatives, sequela on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character S in T42.0X5S mean?
The final character S makes this a sequela code: it reports a lingering problem that remains after the adverse effect of hydantoin derivatives itself has resolved, not the original event.
What MS-DRG does T42.0X5S group to?
On inpatient claims, adverse effect of hydantoin derivatives, sequela maps to MS-DRG 922, 923, with relative weights from 1.0177 to 1.7494 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T42.0X5S be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of hydantoin derivatives, sequela describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
Is T42.0X5S exempt from POA reporting?
Yes. CMS lists this code among those exempt from present on admission reporting, so hospitals do not assign a POA indicator for adverse effect of hydantoin derivatives, sequela on inpatient claims.
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.
