Phenytoin's therapeutic plasma level is within which range?

• A. 5-15 mcg/mL
• B. 10-20 mcg/mL
• C. 20-30 mcg/mL
• D. 40-60 mcg/mL

Answer: (B)

Phenytoin has a narrow therapeutic window, so keeping its concentration in a specific range is crucial for effectively controlling seizures while minimizing toxicity. The typical therapeutic plasma level for total phenytoin is about 10–20 mcg/mL. Staying within this range tends to provide seizure control without excessive CNS side effects.

A key reason this range is preferred is phenytoin’s binding and metabolism characteristics. It is highly protein-bound, so total levels can be misleading in patients with low albumin; in such cases the free (active) portion may be higher than the total level suggests, and clinicians may use adjusted total levels or measure free phenytoin to guide dosing. Phenytoin also has saturable (nonlinear) metabolism, meaning small increases in dose can produce disproportionately large rises in blood concentration, reinforcing the need for careful monitoring and trough level checks.

Levels below the range are often subtherapeutic and may not adequately prevent seizures, while levels above the range increase the risk of toxicity (such as ataxia, nystagmus, and confusion).