Known brand names include Dilantin and Phenytek.
An antiepileptic (anticonvulsant) taken by mouth as extended-release capsules. It treats tonic-clonic and temporal lobe seizures and prevents or treats seizures during or after neurosurgery. Blood levels guide dosing because small dose changes can cause big level changes.
Labeled use. Tonic-clonic and psychomotor (temporal lobe) seizures, and seizures during or after neurosurgery.
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Phenytoin is inexpensive and effective for focal and generalized tonic-clonic seizures, and IV forms are used after benzodiazepines in status epilepticus. Newer drugs such as levetiracetam or lamotrigine are often preferred for long-term therapy because phenytoin has nonlinear kinetics, many interactions, cosmetic effects, and teratogenic risk. It is not effective for absence seizures.
Thought to block voltage-dependent sodium channels, which limits sustained high-frequency neuronal firing. It is highly protein bound and metabolized by CYP2C9 and CYP2C19. Metabolism saturates at higher levels, so a small dose increase can raise levels sharply. It is also a potent inducer of liver drug-metabolizing enzymes. Half-life averages 22 hours.
Narrow therapeutic index. The label sets a target total serum level of 10 to 20 mcg/mL and warns that levels sustained above that range cause toxicity.
Doses come from the FDA label. Study tool, not for clinical decisions.
No set dose change, but the unbound fraction rises in renal disease. Base monitoring on unbound (free) phenytoin levels. Avoid the oral loading regimen.
None on the oral Dilantin extended capsule label. Withdrawal seizures, severe skin reactions, and hepatotoxicity are labeled warnings.
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No antidote. Early overdose signs are nystagmus, ataxia, and slurred speech, progressing to lethargy, coma, hypotension, bradycardia, and cardiac arrest. Support breathing and circulation; hemodialysis can be considered. Consider other CNS depressants such as alcohol.
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May cause fetal harm, including oral clefts, heart defects, and fetal hydantoin syndrome (facial and skull changes, small nails and digits, growth problems, cognitive deficits). Levels often fall in pregnancy, so monitor unbound levels and adjust. Newborns can have a vitamin K-related bleeding disorder; give vitamin K to the mother before delivery and to the baby after birth.
Phenytoin passes into breast milk. Weigh the benefits of breastfeeding against the mother's need for the drug and possible effects on the infant.
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Start 5 mg/kg/day in two or three equal doses, up to 300 mg daily. Usual maintenance is 4 to 8 mg/kg/day. Children over 6 years and teens may need the minimum adult dose of 300 mg/day.