2026 ICD-10-CM Diagnosis Code T43.226AUnderdosing of selective serotonin reuptake inhibitors, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T43

ICD-10-CM T43.226A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T43.226A is a billable ICD-10-CM diagnosis code for underdosing of selective serotonin reuptake inhibitors, 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
T43.226A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of selective serotonin reuptake inhibitors, initial encounter
Short Description
Underdosing of selective serotonin reuptake inhibitors, init
Parent Code
T43.226
Underdosing of selective serotonin reuptake inhibitors

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 CodeT43.226AUnderdosing of selective serotonin reuptake inhibitors, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T43.226A.

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

  • Citalopram

    a furancarbonitrile that is one of the selective serotonin reuptake inhibitors used as an antidepressant. the drug is also effective in reducing ethanol uptake in alcoholics and is used in depressed patients who also suffer from tardive dyskinesia in preference to tricyclic antidepressants, which aggravate dyskinesia.

  • Fluoxetine

    the first highly specific serotonin uptake inhibitor. it is used as an antidepressant and often has a more acceptable side-effects profile than traditional antidepressants.

  • Fluvoxamine

    a selective serotonin reuptake inhibitor that is used in the treatment of depression and a variety of anxiety disorders.

  • Zimeldine

    one of the selective serotonin reuptake inhibitors formerly used for depression but was withdrawn worldwide in september 1983 because of the risk of guillain-barre syndrome associated with its use. (from martindale, the extra pharmacopoeia, 29th ed, p385)

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
CitalopramT43.221T43.222T43.223T43.224T43.225T43.226
FemoxetineT43.221T43.222T43.223T43.224T43.225T43.226
FluoxetineT43.221T43.222T43.223T43.224T43.225T43.226
FluvoxamineT43.221T43.222T43.223T43.224T43.225T43.226
IndalpineT43.221T43.222T43.223T43.224T43.225T43.226
ZimeldineT43.221T43.222T43.223T43.224T43.225T43.226

Patient EducationClinical

Antidepressants

Antidepressants are prescription medicines to treat depression. Depression is more than feeling a little sad or "blue" for a few days. It's a very common, serious medical illness that affects your mood and general mental health It can make you feel tired, hopeless, worried, or fearful. It can change your thinking, sleeping, and eating.

The full article covers:

  • What are antidepressants?
  • Are antidepressants used for other conditions?
  • What are the different types of antidepressants?
  • Which type of antidepressant is right for me?
  • How long do antidepressants take to work?
  • How long will I need to take an antidepressant?
  • What are the side effects of antidepressants?
  • What can I do to take antidepressants safely?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert T43.226A to ICD-9-CMHistory

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

ICD-9-CM
No Map There is no ICD-9 equivalent for this code.

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 T43.226AOverview

Is T43.226A (Underdosing of selective serotonin reuptake inhibitors) a billable code?

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

What does the 7th character A in T43.226A mean?

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

Can T43.226A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of selective serotonin reuptake inhibitors, 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.