2026 ICD-10-CM Diagnosis Code T40.3X5AAdverse effect of methadone, initial encounter
T40.3X5A is a billable ICD-10-CM diagnosis code for adverse effect of methadone, 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 methadone adverse reaction. 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
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T40.3X5A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Methadone 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 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.
Clinical InformationClinical
Methadone
a synthetic opioid that is used as the hydrochloride. it is an opioid analgesic that is primarily a mu-opioid agonist. it has actions and uses similar to those of morphine. (from martindale, the extra pharmacopoeia, 30th ed, p1082-3)
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 |
|---|---|---|---|---|---|---|
| Amidone | T40.3X1 | T40.3X2 | T40.3X3 | T40.3X4 | T40.3X5 | T40.3X6 |
| Dolophine | T40.3X1 | T40.3X2 | T40.3X3 | T40.3X4 | T40.3X5 | T40.3X6 |
| Levo-iso-methadone | T40.3X1 | T40.3X2 | T40.3X3 | T40.3X4 | T40.3X5 | T40.3X6 |
| Methadone | T40.3X1 | T40.3X2 | T40.3X3 | T40.3X4 | T40.3X5 | T40.3X6 |
| Physeptone | T40.3X1 | T40.3X2 | T40.3X3 | T40.3X4 | T40.3X5 | T40.3X6 |
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 T40.3X5A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T40.3X5AOverview
Is T40.3X5A (Adverse effect of methadone) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of methadone, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T40.3X5A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of methadone, such as an emergency visit or first evaluation.
What MS-DRG does T40.3X5A group to?
On inpatient claims, adverse effect of methadone, 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 T40.3X5A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of methadone, 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 T40.3X5A?
Under the General Equivalence Mappings, adverse effect of methadone, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E935.1 (adv eff methadone). 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.
