Question
Phenytoin
Answer
Phenytoin is a Na⁺ channel-blocking antiepileptic.
MOA: phenytoin → binds and stabilizes the inactivated state of voltage-gated Na⁺ channels → use-dependent block of sustained high-frequency neuronal firing, without affecting normal single action potentials.
Uses: generalized tonic-clonic seizures, focal (partial) seizures, status epilepticus (IV/fosphenytoin, after first-line benzodiazepine). Ineffective, and can worsen, absence and myoclonic seizures.
Adverse effects: gingival hyperplasia (a distinctive, characteristic effect), hirsutism and coarsened facial features with long-term use, cerebellar signs (nystagmus, ataxia) in toxicity, megaloblastic anemia (folate metabolism interference), Stevens-Johnson syndrome/toxic epidermal necrolysis, teratogenicity (fetal hydantoin syndrome), and zero-order kinetics at therapeutic doses — a small dose increment can produce a disproportionate rise in plasma level and toxicity.