PharmGKB summary: phenytoin pathway.

Phenytoin is one of the most widely prescribed antiepileptic drugs (AEDs) in the USA (approximately 52% of AED prescriptions compared with 19% for valproic acid, 11% carbamazepine, and 7% phenobarbital) [1]. It has a narrow therapeutic range and wide interindividual variability in clearance and, as such, therapeutic drug monitoring is often necessary. Adverse effects of phenytoin range from minor (e.g. gingival hyperplasia) to severe and life threatening [e.g. Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN)], and teratogenic (e.g. birth defects).