Initiating Dilantin (Phenytoin) Therapy
The recommended protocol for initiating Dilantin (phenytoin) is an intravenous loading dose of 18-20 mg/kg at a maximum rate of 50 mg/min in adults, with close cardiac monitoring. 1
Intravenous Loading Dose Protocol
IV Administration
- Loading dose: 18-20 mg/kg IV (maximum 1000 mg) 2, 1
- Maximum infusion rate: 50 mg/min in adults, 1-3 mg/kg/min in children 1
- Dilution: Use normal saline only (incompatible with glucose-containing solutions) 1
- Equipment: Use in-line filter and infusion pump 2
Monitoring During IV Administration
- Continuous cardiac monitoring is essential
- Monitor blood pressure and respiratory function
- Reduce infusion rate if heart rate decreases by ≥10 beats/min
- Watch for adverse effects: hypotension, bradyarrhythmias, cardiac arrest, and extravasation injuries 2
Oral Loading Dose Protocol (When IV Not Feasible)
Oral Administration
- Loading dose: 1 gram divided into three doses (400 mg, 300 mg, 300 mg) administered at two-hour intervals 3
- Setting: Should be reserved for patients in a clinic or hospital setting where phenytoin serum levels can be closely monitored 3
- Contraindications: Patients with history of renal or liver disease should not receive oral loading regimen 3
Maintenance Dosing
Adult Maintenance Dosing
- Begin maintenance dosing 24 hours after loading dose 3
- Typical regimen: 300 mg daily (100 mg three times daily) 3
- Adjustment range: Can increase up to 200 mg three times daily if necessary 3
- Once-daily option: If seizure control is established with divided doses, may consider 300 mg once daily using extended phenytoin sodium capsules only 3
Pediatric Maintenance Dosing
- Initial dose: 5 mg/kg/day in 2-3 divided doses 3
- Maintenance dose: 4-8 mg/kg/day 3
- Maximum: Up to 300 mg daily 3
- Children >6 years and adolescents may require adult dosing 3
Therapeutic Monitoring
- Target serum level: 10-20 mcg/mL (total) or 1-2 mcg/mL (unbound) 1, 3
- Time to steady-state: 7-10 days to achieve steady-state blood levels 3
- Adjustment timing: Changes in dosage should not occur at intervals shorter than 7-10 days 3
- Monitoring frequency: Frequent serum level determinations after loading dose 3
Special Considerations
- When switching between IV and oral formulations, maintain the same total daily dose, accounting for bioavailability differences (IV is 100% bioavailable, oral is approximately 90%) 1
- Monitor unbound phenytoin concentrations rather than total levels in patients with hypoalbuminemia, renal failure, or other conditions affecting protein binding 1
- Elderly patients require lower or less frequent dosing due to decreased clearance 1
- During pregnancy, monitor unbound phenytoin concentrations due to potential changes in protein binding 1
Common Pitfalls to Avoid
- Infusion rate errors: Exceeding the maximum infusion rate can cause cardiac arrhythmias and hypotension
- Dilution errors: Never mix phenytoin with glucose solutions
- Inadequate monitoring: Failure to monitor cardiac function during IV administration
- Formulation confusion: Different phenytoin products have different absorption characteristics; do not interchange brands without monitoring levels 3
- Dosage adjustment errors: Making changes more frequently than every 7-10 days can lead to toxicity due to the long half-life
If phenytoin fails or is contraindicated, alternative agents include valproate (20-30 mg/kg IV), levetiracetam (40-60 mg/kg IV), or phenobarbital (10-20 mg/kg IV) 1.