Adverse Effects vs. Poisoning in ICD-10-CM 

(T36–T50)

Why do we need to know this? There are different sequencing rules for each. ICD-10-CM guidelines provides greater detail in defining each term and providing examples. The same drug and same symptoms may appear in both categories — the difference is how the substance was used.

Adverse Effect Coding Guidelines and Sequencing

Definition-A reaction occurs when the correct drug is:

• Prescribed correctly
• Administered correctly
• Taken as directed

Sequencing-Nature of the adverse effect:

• Tachycardia
• Delirium
• GI hemorrhage
• Vomiting
• Hypokalemia
• Hepatitis
• Renal failure
• Respiratory failure

T36–T50 code with 5th/6th character “5”

Example: Patient took her prescribed dose of penicillin and has severe nausea and vomiting. Physician final diagnosis is nausea and vomiting due to adverse effect of penicillin.

• R11.2 Nausea with vomiting
• T36.0X5A Adverse Effect of Penicillin, initial encounter

Key Indicator: Everything about the drug use was correct — the reaction is the issue.

Poisoning Coding Guidelines and Sequencing

Definition-The substance was used incorrectly in any way.

Examples of Poisoning Scenarios

• Prescription error
• Wrong drug taken
• Wrong route
• Overdose (intentional or unintentional)
• Taking someone else’s medication
• Nonprescribed drug taken with a prescribed drug
• Drug–alcohol interaction

Sequencing-Poisoning code (T36–T50) first, using correct intent character:

• 1 = Accidental
• 2 = Intentional self-harm
• 3 = Assault
• 4 = Undetermined
• If intent unclear → default to accidental

All manifestations next

• Seizures
• Hypotension
• Altered mental status

Example: Patient comes in with altered mental status. He states he accidentally took two OxyContin tablets at the same time instead of one as prescribed.

• T48.0X1A Poisoning by oxytocic drugs, accidental (unintentional), initial encounter
• R41.82 Altered mental status

Key Indicator: Something about the drug use was incorrect, regardless of symptoms.

Quick Coding Memory Rules for Adverse Effects vs. Poisoning

Quick Memory Rules

Adverse Effect:
Reaction first → drug second
Correct use = adverse effect

Poisoning:
Drug first → reaction second
Incorrect use = poisoning

Documentation Focus Points for Accurate Drug Coding

• Was the drug used as prescribed or incorrectly?
• Was there an error, overdose, or interaction?
• What manifestations occurred?
• Is the intent documented?

Melanie Perrault, RHIA, CDIP, CCS 

Senior Consultant, Audit at UASI

Melanie Perrault is a Senior Consultant in Quality at UASI, with extensive expertise in inpatient coding, CDI, and documentation integrity. She brings a strong educator’s mindset to her coding tips, helping coders and CDI teams strengthen clinical clarity, accuracy, and audit-ready compliance. 

Works Cited

American Hospital Association. AHA Coding Handbook for ICD-10-CM and ICD-10-PCS. 2026. Chapter 31: Poisoning, Toxic Effects, Adverse Effects, and Underdosing of Drugs. Available here: https://www.aha.org/coding-clinic/coding-handbook

Centers for Disease Control and Prevention, National Center for Health Statistics. ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026. Chapter 19: Injury, Poisoning, and Certain Other Consequences of External Causes (S00–T88), Section on Poisoning, Adverse Effects, and Underdosing (T36–T50). Available here: https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2026/ICD-10-CM-October-2025-Guidelines.pdf

American Academy of Professional Coders (AAPC). Verhovshek, J. Poisoning, Adverse Effect, and Underdosing in ICD-10-CM.
Available here: https://www.aapc.com/blog/44094-poisoning-adverse-effect-underdosing-icd-10/