What is the therapeutic range for phenytoin?

• A. 5-15 mg/L
• B. 10-20 mg/L
• C. 20-30 mg/L
• D. 30-40 mg/L

Answer: (B)

Phenytoin has a narrow therapeutic index, so keeping its concentration within a specific range is essential to balance seizure control with toxicity. For most adults, the total phenytoin level target is 10–20 mg/L. This range is high enough to provide anticonvulsant effect but low enough to minimize dose-related adverse effects. Levels below 10 mg/L are often subtherapeutic and may fail to prevent seizures, while levels above about 20–30 mg/L increase the risk of neurological toxicity such as nystagmus, ataxia, impaired coordination, and confusion.

Remember that phenytoin is highly protein bound, so the total level can be misleading in patients with low albumin or those taking interacting drugs; the free, active drug fraction may be higher even if the total level looks within range. In such situations, clinical assessment and sometimes measurement of free phenytoin are important for accurate interpretation.