2026 ICD-10-CM Diagnosis Code T43.656AUnderdosing of methamphetamines, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T43

ICD-10-CM T43.656A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T43.656A is a billable ICD-10-CM diagnosis code for underdosing of methamphetamines, 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
T43.656A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of methamphetamines, initial encounter
Short Description
Underdosing of methamphetamines, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T43.656
Underdosing of methamphetamines

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
CategoryT43Poisoning by, adverse effect of and underdosing of psychotropic drugs, not elsewhere classified
This CodeT43.656AUnderdosing of methamphetamines, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T43.656A.

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 psychotropic drugs, not elsewhere classified (T43). 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

  • Methamphetamine

    a central nervous system stimulant and sympathomimetic with actions and uses similar to dextroamphetamine. the smokable form is a drug of abuse and is referred to as crank, crystal, crystal meth, ice, and speed.

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
DesoxyephedrineT43.651T43.652T43.653T43.654T43.655T43.656
MetamfetamineT43.651T43.652T43.653T43.654T43.655T43.656
MethamphetamineT43.651T43.652T43.653T43.654T43.655T43.656
MethedrineT43.651T43.652T43.653T43.654T43.655T43.656
MethylamphetamineT43.651T43.652T43.653T43.654T43.655T43.656
SpeedT43.651T43.652T43.653T43.654T43.655T43.656

Code History & ChangesHistory

Replacement T43.656A replaces the following previously assigned code(s):

  • T43.626A - Underdosing of amphetamines, initial encounter
FY 2023AddedAdded to the ICD-10-CM code setEffective October 1, 2022.
FY 2024–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T43.656AOverview

Is T43.656A (Underdosing of methamphetamines) a billable code?

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

What does the 7th character A in T43.656A mean?

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

Can T43.656A be a principal diagnosis?

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