2027 ICD-10-CM Diagnosis Code T40.416AUnderdosing of fentanyl or fentanyl analogs, initial encounter
T40.416A is a billable ICD-10-CM diagnosis code for underdosing of fentanyl or fentanyl analogs, 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 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 951. The code is not accepted as a principal diagnosis by the Medicare Code Editor. Coders also document this condition as fentanyl taken at lower dose than instructed. 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 T40.416A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Fentanyl taken at lower dose than instructed
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 2027, published by CMS and the National Center for Health Statistics.
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to T40.416A: its own notes plus those printed at T40, block T36-T50, and Chapter 19. A note printed at a category, block or chapter applies to every code under it.
Includes
Further defines, or gives examples of, the content of the category.
- adverse effect of correct substance properly administered
- poisoning by overdose of substance
- poisoning by wrong substance given or taken in error
- underdosing by (inadvertently) (deliberately) taking less substance than prescribed or instructed
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
- toxic reaction to local anesthesia in pregnancy (O29.3-)
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
Code First
The underlying condition named in the note is sequenced before this code.
Use Additional Code
Add a second code, after this one, to fully describe the condition.
- code to identify any retained foreign body, if applicable (Z18.-)
Notes
General instructions on how these codes are used.
- Use secondary code(s) from Chapter 20, External causes of morbidity, to indicate cause of injury. Codes within the T section that include the external cause do not require an additional external cause code
- The chapter uses the S-section for coding different types of injuries related to single body regions and the T-section to cover injuries to unspecified body regions as well as poisoning and certain other consequences of external causes.
7th Character
Every code in the category takes a 7th character from the listed values, with the placeholder X filling any empty positions before it.
- The appropriate 7th character is to be added to each code from category T40
- Ainitial encounter this code
- Dsubsequent encounter
- Ssequela
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Alfentanil
a short-acting opioid anesthetic and analgesic derivative of fentanyl. it produces an early peak analgesic effect and fast recovery of consciousness. alfentanil is effective as an anesthetic during surgery, for supplementation of analgesia during surgical procedures, and as an analgesic for critically ill patients.Fentanyl
a potent narcotic analgesic, abuse of which leads to habituation or addiction. it is primarily a mu-opioid agonist. fentanyl is also used as an adjunct to general anesthetics, and as an anesthetic for induction and maintenance. (from martindale, the extra pharmacopoeia, 30th ed, p1078)Sufentanil
an opioid analgesic that is used as an adjunct in anesthesia, in balanced anesthesia, and as a primary anesthetic agent.
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 T40.416A replaces the following previously assigned code(s):
- T40.4X6A - Underdosing of other synthetic narcotics, initial encounter
Questions About T40.416AOverview
What is the ICD-10 code for underdosing of fentanyl or fentanyl analogs, initial encounter?
The ICD-10-CM code for underdosing of fentanyl or fentanyl analogs, initial encounter is T40.416A (sometimes written as T40416A). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is T40.416A (Underdosing of fentanyl or fentanyl analogs) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of fentanyl or fentanyl analogs, initial encounter on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What does the 7th character A in T40.416A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of fentanyl or fentanyl analogs, such as an emergency visit or first evaluation.
What MS-DRG does T40.416A group to?
On inpatient claims, underdosing of fentanyl or fentanyl analogs, initial encounter maps to MS-DRG 951 (other Factors Influencing Health Status), which carries a relative weight of 0.5574. Higher weights mean higher Medicare reimbursement.
Can T40.416A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of fentanyl or fentanyl analogs, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
