What is the recommended dosage and management for patients on Dilantin (phenytoin)?

Phenytoin (Dilantin) Dosing and Management Recommendations

The recommended dosage for phenytoin in adults is a loading dose of 15-20 mg/kg administered intravenously at a maximum rate of 50 mg/minute, followed by a maintenance dose of 300-400 mg/day (4-6 mg/kg/day) divided into 1-3 doses. 1, 2

Loading Dose Administration

• For IV loading, administer 18 mg/kg at a maximum rate of 50 mg/minute to achieve therapeutic levels (>10 mcg/mL) immediately after infusion 3, 4
• For oral loading, administer 20 mg/kg divided in maximum doses of 400 mg every 2 hours, which takes >5 hours to reach therapeutic levels 3, 2
• Fosphenytoin can be administered at a faster rate (150 mg/minute) with fewer adverse events compared to IV phenytoin 3, 5
• In emergency settings, a 1 gram oral loading dose can be divided into three doses (400 mg, 300 mg, 300 mg) administered at two-hour intervals, followed by normal maintenance dosage 24 hours later 2

Maintenance Dosing

• Adult maintenance dose: 300-400 mg/day (4-6 mg/kg/day), typically divided into 3-4 doses daily 2
• Once-daily dosing with 300 mg may be considered if seizure control is established with divided doses, but only with Dilantin extended phenytoin sodium capsules 2
• Pediatric dosing: Initially 5 mg/kg/day in 2-3 divided doses, with maintenance of 4-8 mg/kg/day (maximum 300 mg daily) 2
• Therapeutic serum levels are typically 10-20 mcg/mL, requiring 7-10 days to achieve steady-state with conventional dosing 2, 6

Administration Considerations

• IV administration requires:
  • Maximum infusion rate of 50 mg/minute to minimize cardiovascular adverse effects 3, 7
  • Use of a filter and infusion pump 3
  • Dilution in normal saline (not glucose solutions) to prevent precipitation 5
• When changing between formulations (extended capsules, prompt capsules, suspension) or between sodium salt and free acid forms, serum level monitoring is necessary due to differences in bioavailability 2, 6

Monitoring Recommendations

• Monitor serum drug levels to ensure therapeutic range (10-20 mcg/mL) 2, 4
• For loading doses, check levels immediately after infusion and at 4,12, and 24 hours 4
• Adjust dosage at intervals no shorter than 7-10 days to allow achievement of steady-state levels 2
• Monitor cardiac rhythm and vital signs during IV administration, especially in elderly patients or those with cardiovascular comorbidities 7

Adverse Effects to Monitor

• IV administration: hypotension, bradycardia, cardiac arrhythmias, and local tissue reactions/extravasation injuries 3, 7
• Neurological: ataxia, nystagmus, tremor, somnolence, and peripheral neuropathy 1
• Drug interactions: increased levels of phenytoin with disulfiram (Antabuse) 3
• Phenytoin may increase levels of other medications, including phenytoin (Dilantin) 3

Special Populations

• Elderly patients and those with cardiovascular disease require slower infusion rates and careful monitoring 7
• Patients with renal or hepatic disease should not receive oral loading regimens 2
• For status epilepticus refractory to benzodiazepines, IV phenytoin, fosphenytoin, or valproate may be administered 3

Cautions and Pitfalls

• Rapid IV administration (>50 mg/min) is associated with increased mortality and should be strictly avoided 7
• Bioavailability varies between individuals (57.7-85.6%), making serum level monitoring essential for dose adjustments 6
• Phenytoin exhibits non-linear pharmacokinetics, meaning small dose increases can lead to disproportionate increases in serum levels 6
• Delayed toxicity including skin rash may develop 8-10 days after initiating therapy, particularly with loading dose regimens 8