What is the recommended dosing regimen for Dilantin (phenytoin), including loading and maintenance doses for adults and dose adjustments for elderly and pediatric patients?

Dilantin (Phenytoin) Dosing

For adult loading, use 18-20 mg/kg IV at a maximum rate of 50 mg/min, or 20 mg/kg orally divided into maximum 400 mg doses every 2 hours; for pediatric patients use 20 mg/kg IV (neonates 10 mg/kg), and for elderly patients, daily maintenance doses should not exceed 300 mg to avoid toxicity. 1, 2

Adult Dosing

Loading Dose

• IV route: 18-20 mg/kg infused at maximum rate of 50 mg/min 1

  • Faster to achieve therapeutic levels but carries higher risk of hypotension, bradyarrhythmias, cardiac arrest, and extravasation injuries
  • Requires cardiac monitoring, infusion pump, and filter
  • Dilute in normal saline only (incompatible with glucose-containing solutions) 2
  • Reduce infusion rate if heart rate decreases by 10 beats per minute 2

• Oral route: 20 mg/kg divided into maximum 400 mg doses every 2 hours 1

  • Takes >5 hours to reach therapeutic levels
  • Cheaper and safer than IV with no significant difference in seizure recurrence rates 1
  • Therapeutic levels (10 mcg/mL) typically achieved within 2-3 hours 3

Maintenance Dosing

• Start at 4-7 mg/kg/day divided into appropriate intervals 4
• Adjust based on clinical response and serum levels
• Begin maintenance therapy 12 hours after loading dose to prevent subtherapeutic levels 5, 6

Pediatric Dosing

Loading Dose

• Children: 20 mg/kg IV over 10-20 minutes (maximum 1000 mg) 2
• Neonates: 10 mg/kg IV 2
  • Neonates have increased toxicity risk due to decreased protein binding
  • Phenobarbital is preferred in this age group 2

Maintenance Dosing

• Infusion rate should not exceed 1 mg/kg/min 2
• For fosphenytoin: maintenance dose of 5-7.5 mg/kg every 12 hours is superior to every 24 hours 6
• Clearance reaches adult levels by age 4 years 7

Critical Monitoring Points

• Measure serum phenytoin at 2 hours and 12 hours after loading dose 5, 6
• A 2-hour level <16 mcg/mL predicts subtherapeutic levels by 12 hours and requires earlier maintenance dosing 5
• Prepare for respiratory support as apnea risk increases when combined with other sedatives 2

Elderly Dosing

Daily doses should not exceed 300 mg to avoid potentially toxic levels 8

• Doses of 350 mg/day or more produce higher and potentially toxic levels in elderly patients
• Pharmacokinetic parameters (Vmax and Km) are similar to younger patients, but clearance decreases after age 20 8, 7
• Start conservatively and monitor closely

Important Caveats and Pitfalls

Fosphenytoin Alternative

• Preferred over phenytoin when available 1, 2
• Dose: 18 PE/kg IV at maximum rate of 150 PE/min
• Fewer adverse cardiac effects in head-to-head comparisons 1
• Can be given IM if IV access unavailable 1

Status Epilepticus Context

When phenytoin fails after benzodiazepines in refractory status epilepticus, the evidence increasingly favors alternatives like levetiracetam or valproate over continuing with phenytoin 1, 9. Phenytoin's narrow mechanism of action, complex pharmacokinetics, and adverse effect profile make it less desirable as first-line therapy after benzodiazepines fail 9.

Genetic Considerations

• CYP2C9 poor metabolizers require dose reductions to avoid toxicity 10
• HLA-B*15:02 carriers have increased risk of Stevens-Johnson syndrome and toxic epidermal necrolysis 10
• Consider pharmacogenetic testing when available

Administration Requirements

• Never mix with glucose-containing solutions - causes precipitation 2
• Monitor cardiac rhythm continuously during IV administration 2
• Have phentolamine available (0.1-0.2 mg/kg up to 10 mg) for extravasation injuries 2
• Oxygen saturation monitoring is mandatory 2