Prescribing-label excerpts
Phenytoin
Reference for: Phenytoin Sodium. Source presentation: tablet, chewable. Source route: oral.
Mechanism of action
The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a reduction in sustained high-frequency neuronal discharges.
Pharmacokinetics
Absorption — For Phenytoin Infatabs, peak levels occur 1½ to 3 hours after administration. Steady-state therapeutic levels are achieved at least 7 to 10 days (5 to 7 half-lives) after initiation of therapy with recommended doses of 300 mg/day. When serum level determinations are necessary, they should be obtained at least 5 to 7 half-lives after treatment initiation, dosage change, or addition or subtraction of another drug to the regimen so that equilibrium or steady-state will have been achieved.
Clinical studies show that chewed and unchewed Phenytoin Infatabs are bioequivalent, yield approximately equivalent serum levels, and are more rapidly absorbed than 100 mg DILANTIN (phenytoin sodium) extended capsules.
Distribution — Phenytoin is extensively bound to serum plasma proteins.
Elimination — Clinical studies using Phenytoin Infatabs have shown an average plasma half-life of 14 hours with a range of 7 to 29 hours.
Drug Interaction Studies — Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19.
Phenytoin is a potent inducer of hepatic drug-metabolizing enzymes [see Drug Interactions (7.1, 7.2)].
Selected passages from the cited U.S. prescribing label. The studies concern the source product and populations named in each passage; they do not establish Walter-product bioequivalence, an approved indication, or the kinetics of another fixed combination. Tables and the full prescribing information remain available in the source.
Source: DailyMed: Phenytoin — tablet, chewable
Reference accessed . Label revision: 2026-05-22.
