2026 ICD-10-CM Diagnosis Code T40.605AAdverse effect of unspecified narcotics, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T40

ICD-10-CM T40.605A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T40.605A is a billable ICD-10-CM diagnosis code for adverse effect of unspecified narcotics, 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. Coders also document this condition as narcotic embryopathy. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Adverse effects of drugs and medicaments, initial encounter; and Opioid-related disorders.

Code Identity

ICD-10-CM Code
T40.605A
Billable Status
Yes — Valid for Submission
Code Describes
Adverse effect of unspecified narcotics, initial encounter
Short Description
Adverse effect of unspecified narcotics, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T40.605
Adverse effect of unspecified narcotics

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 CodeT40.605AAdverse effect of unspecified narcotics, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T40.605A.

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.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Narcotic embryopathy

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 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 INJ028
Adverse effects of drugs and medicaments, initial encounter
Default principal diagnosis: inpatient No · outpatient Yes
CCSR MBD018
Opioid-related disorders
Default principal diagnosis: inpatient No · outpatient No

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
Narcotic (drug)T40.601T40.602T40.603T40.604T40.605T40.606
Narcotic (drug)::analgesic NECT40.601T40.602T40.603T40.604T40.605T40.606
Narcotic (drug)::antagonistT40.601T40.602T40.603T40.604T40.605T40.606
Narcotic (drug)::specified NECT40.601T40.602T40.603T40.604T40.605T40.606
Narcotic (drug)::syntheticT40.601T40.602T40.603T40.604T40.605T40.606
Opiate NECT40.601T40.602T40.603T40.604T40.605T40.606
Opiate NEC::antagonistsT40.601T40.602T40.603T40.604T40.605T40.606

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 T40.605A to ICD-9-CMHistory

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

ICD-9-CM
995.29 Adv eff med/biol NEC/NOS
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.
ICD-9-CM
E935.2 Adv eff opiates
ApproximateCombination The match is approximate, and more than one code can be needed to describe the source diagnosis. Confirm with contextual judgment.

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 T40.605AOverview

Is T40.605A (Adverse effect of unspecified narcotics) a billable code?

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

What does the 7th character A in T40.605A mean?

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

What MS-DRG does T40.605A group to?

On inpatient claims, adverse effect of unspecified narcotics, 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 T40.605A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of unspecified narcotics, 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 T40.605A?

Under the General Equivalence Mappings, adverse effect of unspecified narcotics, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E935.2 (adv eff opiates). 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.