Management of Subtherapeutic Phenytoin Levels in Epilepsy

Clinical Decision Algorithm

For patients requiring reloading due to breakthrough seizures, administer 15-20 mg/kg IV at maximum rate of 50 mg/min, requiring continuous cardiac monitoring for bradycardia, arrhythmias, and heart block, and monitor blood pressure for hypotension throughout infusion, according to Praxis Medical Insights 1
Administer 20 mg/kg divided in maximum doses of 400 mg every 2 hours orally, which takes >5 hours to reach therapeutic levels but is safer and cheaper than IV, with no significant difference in seizure recurrence between oral and IV loading, as reported by the Annals of emergency medicine 2

The choice between oral versus parenteral administration is at physician discretion, with evidence not supporting one route over the other for preventing early seizure recurrence, as reported by the Annals of emergency medicine 2 and 3
IV route is faster but has more serious adverse effects, including hypotension, bradyarrhythmias, cardiac arrest, and extravasation injuries, while oral route is significantly cheaper but takes longer, as reported by the Annals of emergency medicine 2 and 4

Consider IV fosphenytoin (18 PE/kg at maximum 150 PE/min) which has fewer adverse events than phenytoin in head-to-head comparison, and is now available as generic with significant cost reduction, as reported by the Annals of emergency medicine 2
Standard maintenance dosing is 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses, as recommended by Praxis Medical Insights 1