Why is therapeutic drug monitoring used for certain antiseizure medications, and which drug often requires it?

• A. To maintain efficacy and minimize toxicity for drugs with narrow therapeutic ranges (e.g., phenytoin, valproic acid).
• B. To measure blood pressure.
• C. To monitor kidney function only.
• D. To measure patient satisfaction.

Answer: (A)

Therapeutic drug monitoring is used to keep drug exposure within a range that reliably controls seizures while minimizing harmful effects. Some antiseizure medications have a narrow therapeutic window and highly variable pharmacokinetics between people, so small changes in dose or factors like liver function, protein levels, or drug interactions can cause big shifts in blood levels. By measuring trough (pre-dose) levels and adjusting the dose, clinicians aim to maintain efficacy and avoid toxicity. Phenytoin and valproic acid are classic examples where monitoring is often necessary because their effectiveness and risk of adverse effects closely track their blood concentrations. Other options don’t fit because they target measurements (like blood pressure, kidney function, or patient satisfaction) that aren’t about keeping antiseizure drug levels within a therapeutic range.