Drug interactions
Phenytoin + Rifampin
Severity: Major
Phenytoin and Rifampin: major 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
Rifampin induces phenytoin metabolism and can lower levels, risking breakthrough seizures. Phenytoin may also reduce rifampin exposure.
What this interaction means in plain words
Phenytoin and Rifampin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Why it happens
Rifampin induces CYP2C9 and CYP2C19, increasing phenytoin clearance.
What to do
Monitor phenytoin levels and seizure control closely and increase the dose as needed. Reduce the dose again, with level monitoring, when rifampin is stopped.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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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 Phenytoin with Rifampin?
MediHelp rates the interaction as major. Phenytoin and Rifampin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Rifampin induces phenytoin metabolism and can lower levels, risking breakthrough seizures. Phenytoin may also reduce rifampin exposure.
How serious is the interaction between Phenytoin and Rifampin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor phenytoin levels and seizure control closely and increase the dose as needed. Reduce the dose again, with level monitoring, when rifampin 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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