Which monitoring parameters are important for patients on chronic phenytoin therapy?

• A. Therapeutic range 40–60 mcg/mL; monitor only for drug interactions.
• B. Therapeutic range 10–20 mcg/mL; monitor for CYP induction and drug interactions; CBC/LFTs; assess gingival changes and coordination.
• C. Therapeutic range 0.5–1 mcg/mL; monitor for platelet counts.
• D. No routine monitoring is required.

Answer: (B)

Phenytoin has a narrow therapeutic index, so keeping its level in the commonly cited range of about 10–20 mcg/mL helps maintain seizure control while avoiding toxicity. Because of this narrow window, ongoing monitoring is essential.

In chronic therapy, you watch for drug interactions since phenytoin is a powerful enzyme inducer of hepatic cytochrome P450 enzymes. That means many other drugs can be metabolized faster and become less effective, so checking for interactions and adjusting other medications as needed is a key part of care.

Labs are also routinely checked: CBC to catch potential hematologic effects (like thrombocytopenia or leukopenia) and LFTs to monitor for liver-related issues. Phenytoin can cause gingival hyperplasia over time and can affect coordination and balance, so clinical assessments of the gums and neuromotor function are part of follow-up.

The other options either give an incorrect therapeutic range, omit essential monitoring (labs, interactions, and physical signs), or suggest no routine monitoring, which is unsafe with this drug.