2026 ICD-10-CM Diagnosis Code T42.3X5AAdverse effect of barbiturates, initial encounter
T42.3X5A is a billable ICD-10-CM diagnosis code for adverse effect of barbiturates, 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) and groups to MS-DRG 917 through 918. 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 Adverse effects of drugs and medicaments, initial encounter.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T42.3X5A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Adverse reaction to barbitone
- Adverse reaction to barbiturate
- Adverse reaction to pentobarbitone
- Amylobarbitone adverse reaction
- Barbiturate antiepileptic adverse reaction
- Butobarbitone adverse reaction
- Cyclobarbitone adverse reaction
- Methylphenobarbitone adverse reaction
- Phenobarbital embryopathy
- Phenobarbitone adverse reaction
- Quinalbarbitone adverse reaction
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
Barbital
a long-acting barbiturate that depresses most metabolic processes at high doses. it is used as a hypnotic and sedative and may induce dependence. barbital is also used in veterinary practice for central nervous system depression.Hexobarbital
a barbiturate that is effective as a hypnotic and sedative.Mephobarbital
a barbiturate that is metabolized to phenobarbital. it has been used for similar purposes, especially in epilepsy, but there is no evidence mephobarbital offers any advantage over phenobarbital.Pentobarbital
a short-acting barbiturate that is effective as a sedative and hypnotic (but not as an anti-anxiety) agent and is usually given orally. it is prescribed more frequently for sleep induction than for sedation but, like similar agents, may lose its effectiveness by the second week of continued administration. (from ama drug evaluations annual, 1994, p236)Phenobarbital
a barbituric acid derivative that acts as a nonselective central nervous system depressant. it potentiates gamma-aminobutyric acid action on gaba-a receptors, and modulates chloride currents through receptor channels. it also inhibits glutamate induced depolarizations.Secobarbital
a barbiturate that is used as a sedative. secobarbital is reported to have no anti-anxiety activity.
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.
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 T42.3X5A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T42.3X5AOverview
Is T42.3X5A (Adverse effect of barbiturates) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of barbiturates, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T42.3X5A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of barbiturates, such as an emergency visit or first evaluation.
What MS-DRG does T42.3X5A group to?
On inpatient claims, adverse effect of barbiturates, initial encounter maps to MS-DRG 917, 918, with relative weights from 0.8571 to 1.5684 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T42.3X5A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of barbiturates, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
What is the ICD-9 equivalent of T42.3X5A?
Under the General Equivalence Mappings, adverse effect of barbiturates, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E937.0 (adv eff barbiturates). The mapping is approximate, so confirm the match fits the documentation.
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.
