2026 ICD-10-CM Diagnosis Code T40.496AUnderdosing of other synthetic narcotics, initial encounter
T40.496A is a billable ICD-10-CM diagnosis code for underdosing of other synthetic narcotics, 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 T40.496A.
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 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
Alphaprodine
an opioid analgesic chemically related to and with an action resembling that of meperidine, but more rapid in onset and of shorter duration. it has been used in obstetrics, as pre-operative medication, for minor surgical procedures, and for dental procedures. (from martindale, the extra pharmacopoeia, 30th ed, p1067)Buprenorphine
a derivative of the opioid alkaloid thebaine that is a more potent and longer lasting analgesic than morphine. it appears to act as a partial agonist at mu and kappa opioid receptors and as an antagonist at delta receptors. the lack of delta-agonist activity has been suggested to account for the observation that buprenorphine tolerance may not develop with chronic use.Buprenorphine, Naloxone Drug Combination
a pharmaceutical preparation that combines buprenorphine, an opioid analgesics with naloxone, a narcotic antagonists to reduce the potential for narcotic dependence in the treatment of pain. it may also be used for opioid substitution therapy.Butorphanol
a synthetic morphinan analgesic with narcotic antagonist action. it is used in the management of severe pain.Dextropropoxyphene
a narcotic analgesic structurally related to methadone. only the dextro-isomer has an analgesic effect; the levo-isomer appears to exert an antitussive effect.Levopropoxyphene
a propionate derivative that is used to suppress coughing.Levorphanol
a narcotic analgesic that may be habit-forming. it is nearly as effective orally as by injection.Meperidine
a narcotic analgesic that can be used for the relief of most types of moderate to severe pain, including postoperative pain and the pain of labor. prolonged use may lead to dependence of the morphine type; withdrawal symptoms appear more rapidly than with morphine and are of shorter duration.Nalbuphine
a narcotic used as a pain medication. it appears to be an agonist at kappa receptors and an antagonist or partial agonist at mu receptors.Pentazocine
the first mixed agonist-antagonist analgesic to be marketed. it is an agonist at the kappa and sigma opioid receptors and has a weak antagonist action at the mu receptor. (from ama drug evaluations annual, 1991, p97)Phenazocine
an opioid analgesic with actions and uses similar to morphine. (from martindale, the extra pharmacopoeia, 30th ed, p1095)
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.
Code History & ChangesHistory
Replacement T40.496A replaces the following previously assigned code(s):
- T40.4X6A - Underdosing of other synthetic narcotics, initial encounter
Questions About T40.496AOverview
Is T40.496A (Underdosing of other synthetic narcotics) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of other synthetic narcotics, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T40.496A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of other synthetic narcotics, such as an emergency visit or first evaluation.
Can T40.496A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of other synthetic narcotics, 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.
