Overview
Phenytoin is an anticonvulsant medication primarily used to control tonic-clonic and complex partial seizures. It works by stabilizing neuronal membranes and reducing seizure activity through sodium channel blockade. While historically important, it has been largely replaced by newer antiepileptic drugs due to its narrow therapeutic index and complex pharmacokinetics.
Mechanism of Action
Phenytoin exerts its anticonvulsant effect by blocking voltage-gated sodium channels in neurons, thereby stabilizing neuronal membranes and preventing the spread of seizure activity. It preferentially inhibits high-frequency neuronal firing without affecting normal neuronal transmission.
Indications
- Treatment of generalized tonic-clonic seizures
- Management of complex partial seizures
- Prevention and treatment of seizures following neurosurgery or traumatic brain injury
Common Doses
- 100 mg capsules
- 200 mg extended-release capsules
- 300 mg extended-release capsules
- 125 mg/5 mL oral suspension
Dosage
Adult loading dose: 15-20 mg/kg IV at ≤50 mg/min. Maintenance: 300-400 mg/day orally in divided doses or once daily with extended-release formulation. Requires therapeutic drug monitoring (target range 10-20 mcg/mL).
Black Box Warning
Black Box Warning: Risk of serious dermatologic reactions including Stevens-Johnson syndrome and toxic epidermal necrolysis
Contraindications
- Hypersensitivity to phenytoin or other hydantoins
- Sinus bradycardia, sinoatrial block, or second/third degree AV block
- Patients with a history of prior phenytoin-induced hepatotoxicity
Side Effects
- Common: Nystagmus, ataxia, slurred speech, dizziness, gingival hyperplasia
- Serious: Stevens-Johnson syndrome, drug rash with eosinophilia and systemic symptoms (DRESS), hepatic necrosis, megaloblastic anemia, peripheral neuropathy
Interactions
- Warfarin: Increased anticoagulant effect
- Oral contraceptives: Reduced contraceptive efficacy
- Valproic acid: Complex bidirectional interactions affecting levels
- Alcohol: Increased CNS depression
- Carbamazepine: Mutual induction of metabolism
Counseling Points
- Take consistently with or without food at same time each day
- Do not crush or chew extended-release capsules
- Report rash, fever, sore throat, or unusual bleeding/bruising immediately
- Regular dental care needed due to gingival hyperplasia risk
- Avoid alcohol consumption
- Carry medical identification indicating phenytoin use