Phenytoin Dosing for a 60-kg Adult
For a 60-kg adult, administer a loading dose of 900–1,200 mg (15–20 mg/kg) intravenously at a maximum rate of 50 mg/minute, followed by a maintenance dose of 300–400 mg/day (5–6.7 mg/kg/day) divided into doses every 6–8 hours or given once daily. 1, 2
Loading Dose Administration
Dose Calculation
- Standard loading dose: 15–20 mg/kg IV 1, 2
- For a 60-kg patient: This translates to 900–1,200 mg total dose 1, 2
- Preferred dose: 18 mg/kg (1,080 mg for 60 kg) achieves therapeutic levels (>10 mcg/mL) in 97% of patients immediately after infusion 3
Infusion Rate and Timing
- Maximum infusion rate: 50 mg/minute in adults 1, 2
- Infusion duration: A 1,000-mg loading dose administered at 50 mg/min requires approximately 20 minutes 2
- Never exceed 50 mg/min: Rapid administration causes hypotension, bradycardia, cardiac arrhythmias, and cardiac arrest 1, 3
Critical Safety Monitoring During Infusion
- Continuous ECG monitoring is mandatory to detect bradycardia, arrhythmias, and heart block 1
- Blood pressure monitoring throughout infusion to detect hypotension 1, 3
- Reduce infusion rate if heart rate decreases by 10 beats/min 1
- Respiratory monitoring for signs of respiratory depression 2
Administration Preparation
- Dilute only in normal saline with final concentration ≥5 mg/mL 1
- Never mix with dextrose solutions: This causes precipitation 1, 3
- Avoid intramuscular administration for acute seizures due to erratic absorption and risk of tissue necrosis; peak levels may require up to 24 hours 2
Maintenance Dose Regimen
Standard Maintenance Dosing
- Initial maintenance dose: 300 mg daily, which can be given as a single daily dose or divided (100 mg three times daily) 4
- Typical maintenance range: 200–700 mg/day (approximately 4–6 mg/kg/day for a 60-kg patient) 1, 4
- Timing: Begin maintenance dosing every 6–8 hours after the loading dose 2
For a 60-kg Patient Specifically
- Standard starting dose: 300 mg/day (5 mg/kg/day) 4
- Upper range if needed: 400 mg/day (6.7 mg/kg/day) 1
- Dose adjustments: Increase incrementally by 100–200 mg/day at weekly intervals if levels are subtherapeutic, with maximum typical adult dose of 1,200 mg/day 4
Therapeutic Level Achievement and Monitoring
Expected Time to Therapeutic Levels
- IV loading: Therapeutic levels (10–20 mcg/mL) achieved within 10 minutes of infusion completion 4
- Confirmation timing: Check level 2–4 hours after completion to confirm sustained therapeutic concentrations 4
- Critical monitoring point: At 12 hours post-loading, approximately 50% of patients may have subtherapeutic levels 4
- 24-hour stability: Most patients (approximately 83%) maintain therapeutic levels at 24 hours after appropriate loading 4
Ongoing Monitoring Strategy
- First level check: 2–4 hours after loading dose completion 4
- Trough levels: Obtain just prior to next scheduled dose to assess therapeutic range 2
- Peak levels: Obtained at time of expected peak concentration to identify threshold for dose-related toxicity 2
- Steady-state timing: Without loading, regular oral maintenance dosing takes 3–7 days to achieve therapeutic levels 4
Important Clinical Caveats
Dose-Related Adverse Effects to Monitor
- Common toxicity signs: Ataxia, nystagmus, tremor, somnolence, and cognitive impairment 1, 4
- Cardiovascular risks: Approximately 2% experience bradycardia and 2% experience arrhythmias 3
- Local reactions: Approximately 15% may experience irritation at infusion site 3
- Extravasation risk: Can cause severe tissue necrosis due to high alkalinity 1
Alternative: Fosphenytoin Advantages
- Faster administration: Can be infused at 150 mg PE/min (three times faster than phenytoin) 1, 4, 3
- Loading dose: 18–20 mg PE/kg IV 1
- Reduced complications: Less hypotension, reduced local irritation, and can be given intramuscularly if needed 1, 3