What is the therapeutic range for phenytoin in serum?

• A. 5-15 mcg/mL
• B. 20-30 mcg/mL
• C. 1-5 mcg/mL
• D. 10-20 mcg/mL

Answer: (D)

Phenytoin requires careful monitoring because its window between effective seizure control and toxicity is narrow. The serum level most clinicians aim for with total phenytoin is 10–20 mcg/mL. Levels lower than this can be subtherapeutic and fail to prevent seizures, while levels higher than this raise the risk of toxicity such as ataxia and nystagmus; values around 20–30 mcg/mL are usually considered toxic. The other ranges fall outside the typical targeted window, and factors like low albumin can make total phenytoin appear low even when clinically appropriate, which is why sometimes free phenytoin or corrected values are considered.