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Phenobarbital

Prescriptionprescriptionoral · inferred

Long-acting barbiturate used for seizure control (all types except absence) and neonatal hyperbilirubinemia. A potent broad-spectrum CYP enzyme and UGT inducer via CAR/PXR activation. Also a GABA-A positive allosteric modulator.

Projected serum levels — 100 mg, once daily

Phenobarbital
Phenobarbital modeled serum levels, 100 mg once daily over 31 days Population-based pharmacokinetic estimate. Steady state reached after approximately 11 days. 0 250 500 750 1k Day 0 Day 8 Day 16 Day 23 Day 31 Time on a regular schedule ≈ steady state · day 11
Phenobarbital modeled serum levels with a loading dose of 300 mg, then 100 mg once daily The first dose is larger so levels approach steady state faster. 0 250 500 750 1k Day 0 Day 8 Day 16 Day 23 Day 31 Time on a regular schedule

Maintenance schedule: 100 mg once daily (oral).

Loading schedule: 300 mg on day 1, then 100 mg once daily — reaching therapeutic levels sooner.

Modeled steady state after ~11 days: peak ≈ 567 mg, trough ≈ 488 mg body load. Population-based estimate over 31 days for a 100 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.

Key facts

Category
Prescription
Route modeled
Oral
Model confidence
inferred
Half-life
4 days
Common dose
100 mg
Suggested maximum
400 mg/day
Reference dose range
25–400 mg (single dose)
Suggested cadence
once daily
Validated against
100 mg oral — Cmax 3 mg/L at 4 h

Documented interactions

  • danger
    7-Hydroxymitragynine (7-OH) + Phenobarbital CYP induction

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

  • danger
    Albendazole (Albenza) + Phenobarbital CYP induction

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

  • danger
    Alfuzosin (Uroxatral) + Phenobarbital CYP induction

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

  • danger
    Alprazolam (Xanax) + Phenobarbital CYP induction

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

  • danger
    Amlodipine (Norvasc) + Phenobarbital CYP induction

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

  • danger
    Anastrozole (Arimidex) + Phenobarbital CYP induction

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

Serum checks 494 modeled interaction pairings for phenobarbital across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.

Research behind this entry

  1. Pharmacokinetic interactions of antiepileptic drugs Patsalos PN et al. · Lancet Neurology, 2013 DOI

    Comprehensive review of AED pharmacokinetic interactions, documenting that phenobarbital is a broad-spectrum inducer of CYP1A2, 2C9, 2C19, 3A4 and UGT enzymes via CAR activation.

1 published study referenced in the app, each with a plain-language summary.