Phenytoin Loading Dose
The loading dose of phenytoin is 15-20 mg/kg intravenously, administered at a maximum rate of 50 mg/min in adults (or 1-3 mg/kg/min in pediatric patients, whichever is slower). 1
Dosing Specifics
Adults
- Loading dose: 15-20 mg/kg IV (most commonly 18-20 mg/kg for status epilepticus)
- Maximum infusion rate: 50 mg/min
- Time to complete: Approximately 20 minutes in a 70-kg patient at maximum rate
- Maintenance: 100 mg IV or PO every 6-8 hours following the load 1
Pediatric Patients
- Loading dose: 15-20 mg/kg IV
- Maximum infusion rate: 1-3 mg/kg/min OR 50 mg/min, whichever is slower
- This achieves therapeutic serum concentrations (10-20 mcg/mL total, 1-2 mcg/mL unbound) 1
Critical Administration Requirements
Cardiac monitoring is mandatory during and after infusion due to risk of:
Rate Adjustments for High-Risk Patients
Reduce infusion rate to 25 mg/min in patients with:
- Age >50 years
- Atherosclerotic cardiovascular disease (ASCVD)
- History of cardiac disease 2
These patients demonstrate significantly more cardiovascular side effects (hypotension and bradycardia) at standard rates.
Administration Technique
- Use large peripheral or central vein with large-gauge catheter
- Verify IV patency with sterile saline flush before administration
- Flush with sterile saline after each injection to prevent local venous irritation
- Can dilute in normal saline (final concentration ≥5 mg/mL)
- Never mix with dextrose solutions - causes precipitation 1
Infusion Method
If using continuous infusion rather than bolus:
- Dilute in normal saline only
- Use 0.22-0.55 micron in-line filter
- Complete within 1-4 hours
- Do not refrigerate diluted mixture 1
Clinical Context
Status Epilepticus
The 10-15 mg/kg loading dose is specifically for status epilepticus in adults, though 15-20 mg/kg is more commonly used and supported by research 1, 3, 4. The higher end (18-20 mg/kg) achieves therapeutic levels more reliably 3, 4, 5.
Alternative Dosing Regimen
For situations requiring slower administration, 15 mg/kg divided into 3 IV injections given 2 hours apart achieves therapeutic concentrations in 90% of patients within 6 hours 4. This approach minimizes cardiovascular risks while maintaining efficacy.
Important Caveats
Do not use IM administration for status epilepticus - peak serum levels may take up to 24 hours 1
Oral loading is safer but slower: Oral phenytoin 20 mg/kg (divided in 400 mg doses every 2 hours) has significantly fewer adverse events than IV administration but requires 5-6 hours to reach therapeutic levels 6. Use IV only when rapid therapeutic levels are essential.
Monitor serum levels: Therapeutic range is 10-20 mcg/mL total (1-2 mcg/mL unbound). In patients with renal/hepatic disease or hypoalbuminemia, monitor unbound phenytoin concentrations 1.