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Phenytoin (Dilantin)

Anticonvulsantprescriptionoral · inferred

Aromatic antiepileptic drug (AED) with nonlinear (Michaelis-Menten) pharmacokinetics due to saturable hepatic hydroxylation. At low doses follows first-order kinetics; at therapeutic doses (>400 mg/day), metabolism saturates causing disproportionate increases in serum concentration with dose increases. Small increases above therapeutic range (10-20 mcg/mL) can cause toxicity. Narrow therapeutic window; therapeutic drug monitoring essential. Potent CYP450 inducer affecting metabolism of warfarin, oral contraceptives, other anticonvulsants. Many drug interactions and adverse effects including gingival hyperplasia, hirsutism, osteoporosis.

Projected serum levels — 300 mg, once daily

Phenytoin (Dilantin)
Phenytoin (Dilantin) modeled serum levels, 300 mg once daily over 15 days Population-based pharmacokinetic estimate. Steady state reached after approximately 3 days. 0 125 250 375 500 Day 0 Day 4 Day 8 Day 11 Day 15 Time on a regular schedule ≈ steady state · day 3
Phenytoin (Dilantin) modeled serum levels with a loading dose of 591 mg, then 300 mg once daily The first dose is larger so levels approach steady state faster. 0 125 250 375 500 Day 0 Day 4 Day 8 Day 11 Day 15 Time on a regular schedule

Maintenance schedule: 300 mg once daily (oral).

Loading schedule: 591 mg on day 1, then 300 mg once daily — reaching therapeutic levels sooner.

Modeled steady state after ~3 days: peak ≈ 414 mg, trough ≈ 252 mg body load. Population-based estimate over 15 days for a 300 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.

Key facts

Category
Anticonvulsant
Route modeled
Oral
Model confidence
inferred
Half-life
22 h
Common dose
300 mg
Suggested maximum
600 mg/day
Reference dose range
100–600 mg (single dose)
Suggested cadence
once daily
Validated against
300 mg oral — Cmax 10 mg/L at 8 h

Documented interactions

  • danger
    7-Hydroxymitragynine (7-OH) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change 7-Hydroxymitragynine (7-OH) AUC by ~0.39x

  • danger
    Albendazole (Albenza) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Albendazole (Albenza) AUC by ~0.44x

  • danger
    Alfuzosin (Uroxatral) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Alfuzosin (Uroxatral) AUC by ~0.33x

  • danger
    Alprazolam (Xanax) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Alprazolam (Xanax) AUC by ~0.33x

  • danger
    Amlodipine (Norvasc) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Amlodipine (Norvasc) AUC by ~0.33x

  • danger
    Anastrozole (Arimidex) + Phenytoin (Dilantin) CYP induction

    Phenytoin (Dilantin) inducer of CYP3A4 predicted to change Anastrozole (Arimidex) AUC by ~0.36x

Serum checks 547 modeled interaction pairings for phenytoin (dilantin) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.

Research behind this entry

  1. Nonlinear kinetics of phenytoin in children Dodson WE et al. · Neurology, 1982 DOI

    Landmark study in 54 children demonstrating nonlinear kinetics of phenytoin with good correlation between dose and serum concentration when Michaelis-Menten model applied (r=0.896).

  2. Michaelis-Menten kinetics and the steady-state serum phenytoin/hydroxyphenytoin ratio Grasela TH Jr et al. · Clinical Pharmacokinetics, 1980 DOI

    Study defining Michaelis-Menten parameters (Km, Vmax) for phenytoin metabolism and explaining how saturation of hepatic enzymes produces nonlinear dose-concentration relationships.

  3. Phenytoin: toxicity and efficacy correlations with plasma concentrations Richens A · Clinical Pharmacokinetics, 1979 DOI

    Classic review establishing therapeutic drug monitoring thresholds for phenytoin (10-20 mcg/mL) and relationship between concentration and clinical efficacy/toxicity.

3 published studies referenced in the app, each with a plain-language summary.