2026 ICD-10-CM Diagnosis Code T43.3X6AUnderdosing of phenothiazine antipsychotics and neuroleptics, initial encounter
T43.3X6A is a billable ICD-10-CM diagnosis code for underdosing of phenothiazine antipsychotics and neuroleptics, 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
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T43.3X6A.
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.
Clinical InformationClinical
Acepromazine
a phenothiazine that is used in the treatment of psychoses.Chlorpromazine
the prototypical phenothiazine antipsychotic drug. like the other drugs in this class chlorpromazine's antipsychotic actions are thought to be due to long-term adaptation by the brain to blocking dopamine receptors. chlorpromazine has several other actions and therapeutic uses, including as an antiemetic and in the treatment of intractable hiccup.Fluphenazine
a phenothiazine used in the treatment of psychoses. its properties and uses are generally similar to those of chlorpromazine.Mesoridazine
a phenothiazine antipsychotic with effects similar to chlorpromazine.Methotrimeprazine
a phenothiazine with pharmacological activity similar to that of both chlorpromazine and promethazine. it has the histamine-antagonist properties of the antihistamines together with central nervous system effects resembling those of chlorpromazine. (from martindale, the extra pharmacopoeia, 30th ed, p604)Perazine
a phenothiazine antipsychotic with actions and uses similar to those of chlorpromazine. extrapyramidal symptoms may be more common than other side effects.Perphenazine
an antipsychotic phenothiazine derivative with actions and uses similar to those of chlorpromazine.Prochlorperazine
a phenothiazine antipsychotic used principally in the treatment of nausea; vomiting; and vertigo. it is more likely than chlorpromazine to cause extrapyramidal disorders. (from martindale, the extra pharmacopoeia, 30th ed, p612)Promazine
a phenothiazine with actions similar to chlorpromazine but with less antipsychotic activity. it is primarily used in short-term treatment of disturbed behavior and as an antiemetic.Thiethylperazine
a dopamine antagonist that is particularly useful in treating the nausea and vomiting associated with anesthesia, mildly emetic cancer chemotherapy agents, radiation therapy, and toxins. this piperazine phenothiazine does not prevent vertigo or motion sickness. (from ama drug evaluations annual, 1994, p457)Thioridazine
a phenothiazine antipsychotic used in the management of phycoses, including schizophrenia.Trifluoperazine
a phenothiazine with actions similar to chlorpromazine. it is used as an antipsychotic and an antiemetic.Triflupromazine
a phenothiazine used as an antipsychotic agent and as an antiemetic.
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
Medication Errors
Medicines treat infectious diseases, prevent problems from chronic diseases, and ease pain. But medicines can also cause harmful reactions if not used correctly. Errors can happen in the hospital, at the health care provider's office, at the pharmacy, or at home. You can help prevent errors by:
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T43.3X6A to ICD-9-CMHistory
Code HistoryHistory
Questions About T43.3X6AOverview
Is T43.3X6A (Underdosing of phenothiazine antipsychotics and neuroleptics) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of phenothiazine antipsychotics and neuroleptics, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T43.3X6A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of phenothiazine antipsychotics and neuroleptics, such as an emergency visit or first evaluation.
Can T43.3X6A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of phenothiazine antipsychotics and neuroleptics, 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.
