Adverse effect of antiviral drugs, initial encounter

This code identifies an undesirable or harmful reaction experienced by a patient due to the therapeutic use of an antiviral medication. This adverse effect is occurring during the period of initial treatment or evaluation for the reaction.

Assign this code when a patient presents with symptoms or conditions directly attributable to an antiviral drug, and this is the first time the adverse effect is being medically addressed. Examples include rash, nausea, or liver enzyme elevation developing after starting an antiviral.

• Always sequence the manifestation of the adverse effect first, followed by this code.
• Requires an additional code for the specific adverse effect (e.g., R21 for rash, K71.0 for toxic liver disease).
• Distinguish from poisoning (T36-T65 with 1st or 2nd character) which implies incorrect dosage or non-therapeutic intent.