Drug interactions
Fosphenytoin + Rifampicin
Severity: Major
Fosphenytoin and Rifampicin: major interaction.
Last updated Sources: US FDA label
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
Rifampicin induces hepatic enzymes and can substantially lower phenytoin concentrations, which may cause breakthrough seizures. The effect can persist for some time after rifampicin is stopped.
What this interaction means in plain words
Fosphenytoin and Rifampicin 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.
Fosphenytoin belongs to Central Nervous System; Rifampicin belongs to Anti Bacterial.
Why it happens
Induction of CYP2C9 and CYP2C19 by rifampicin increases phenytoin metabolism.
What to do
Monitor phenytoin levels and seizure control, and increase the dose as required. Re-check levels and reduce the dose when rifampicin 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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References
Other interactions of Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Other interactions of Rifampicin
- Cabotegravir + Rifampicin Contraindicated
- Delamanid + Rifampicin Contraindicated
- Doravirine + Rifampicin Contraindicated
- Ensitrelvir + Rifampicin Contraindicated
- Fostemsavir + Rifampicin Contraindicated
- Isavuconazole + Rifampicin Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Rifampicin?
MediHelp rates the interaction as major. Fosphenytoin and Rifampicin 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. Rifampicin induces hepatic enzymes and can substantially lower phenytoin concentrations, which may cause breakthrough seizures. The effect can persist for some time after rifampicin is stopped.
How serious is the interaction between Fosphenytoin and Rifampicin?
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, and increase the dose as required. Re-check levels and reduce the dose when rifampicin is stopped.
What other medicines interact with Fosphenytoin?
MediHelp also lists interactions of Fosphenytoin with, for example, Dronedarone, Ledipasvir-sofosbuvir, Lorlatinib. The full list is on the Fosphenytoin page.
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