Phenytoin Administration and Pharmacokinetics

Introduction to Phenytoin Administration

Phenytoin levels can change rapidly with intravenous administration, reaching therapeutic levels within 2-3 hours, while oral administration typically requires 3-10 hours to achieve therapeutic levels, with significant variability between patients 1
The American College of Emergency Physicians recommends that therapeutic levels can be reached within 30 minutes after administration, and all patients given 1,000 mg IV had therapeutic levels by 2 hours, with nearly all patients maintaining therapeutic levels at 24 hours post-infusion with 18 mg/kg dosing 1

Time to therapeutic levels varies based on formulation and patient factors, with 48% of patients reaching therapeutic levels at 3-5 hours after oral loading, and 55% reaching therapeutic levels at 6-10 hours after oral loading 1
In patients with recent seizures, the average time to therapeutic level was 2.04±0.44 hours, and when given in divided doses over 6-12 hours, 80% of patients achieve therapeutic levels at approximately 10.75 hours after the last dose 1

When rapid achievement of therapeutic levels is needed, IV administration is preferred over oral loading, according to the American College of Emergency Physicians 1, 2

The American College of Emergency Physicians recommends that when changing dosage forms or brands, careful monitoring of serum levels is essential, and for loading doses, frequent serum level determinations are necessary 1, 2