2026 ICD-10-CM Diagnosis Code T43.025AAdverse effect of tetracyclic antidepressants, initial encounter
T43.025A is a billable ICD-10-CM diagnosis code for adverse effect of tetracyclic antidepressants, 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 adverse reaction to mirtazapine. 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 T43.025A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Adverse reaction to mirtazapine
- Maprotiline adverse reaction
- Mianserin adverse reaction
- Tetracyclic antidepressant drug 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 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.
Clinical InformationClinical
Maprotiline
a bridged-ring tetracyclic antidepressant that is both mechanistically and functionally similar to the tricyclic antidepressants, including side effects associated with its use.Mianserin
a tetracyclic compound with antidepressant effects. it may cause drowsiness and hematological problems. its mechanism of therapeutic action is not well understood, although it apparently blocks alpha-adrenergic, histamine h1, and some types of serotonin receptors.Mirtazapine
a piperazinoazepine tetracyclic compound that enhances the release of norepinephrine and serotonin through blockage of presynaptic alpha-2 adrenergic receptors. it also blocks both 5-ht2 and 5-ht3 serotonin receptors and is a potent histamine h1 receptor antagonist. it is used for the treatment of depression, and may also be useful for the treatment of anxiety disorders.
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
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 T43.025A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T43.025AOverview
Is T43.025A (Adverse effect of tetracyclic antidepressants) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect of tetracyclic antidepressants, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T43.025A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of tetracyclic antidepressants, such as an emergency visit or first evaluation.
What MS-DRG does T43.025A group to?
On inpatient claims, adverse effect of tetracyclic antidepressants, 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 T43.025A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect of tetracyclic antidepressants, 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 T43.025A?
Under the General Equivalence Mappings, adverse effect of tetracyclic antidepressants, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E939.0 (adv eff antidepressants). 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.
