What is the therapeutic plasma level range for Dilantin (phenytoin)?

• A. 5-15 mcg/mL
• B. 10-20 mcg/mL
• C. 20-30 mcg/mL
• D. 15-25 mcg/mL

Answer: (B)

Phenytoin has a narrow therapeutic window, so its plasma level is used to balance seizure control with the risk of toxicity. The standard therapeutic range for most adults is 10–20 mcg/mL. Staying within this range helps prevent breakthrough seizures while limiting adverse effects. Levels above about 20 mcg/mL are commonly associated with toxicity, such as nystagmus, ataxia, and confusion, whereas levels below 10 mcg/mL may not control seizures effectively. Because phenytoin is highly protein-bound, total levels can be influenced by albumin; in cases of low albumin, the free (active) fraction may be higher, sometimes warranting a free phenytoin measurement.