Drug interactions
Phenobarbital + Phenytoin
Severity: Moderate
Phenobarbital and Phenytoin: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Phenobarbital can raise or lower phenytoin levels, and phenytoin can affect phenobarbital concentrations. Additive sedation also occurs, so seizure control and toxicity can both be affected.
What this interaction means in plain words
Phenobarbital and Phenytoin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Why it happens
Phenobarbital induces or competes for CYP2C9/2C19 metabolism of phenytoin, with additive CNS depression.
What to do
Monitor serum levels of both drugs and sedation, especially after dose changes or when either drug is stopped.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Phenobarbital with Phenytoin?
MediHelp rates the interaction as moderate. Phenobarbital and Phenytoin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Phenobarbital can raise or lower phenytoin levels, and phenytoin can affect phenobarbital concentrations. Additive sedation also occurs, so seizure control and toxicity can both be affected.
How serious is the interaction between Phenobarbital and Phenytoin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor serum levels of both drugs and sedation, especially after dose changes or when either drug is stopped.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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