Drug interactions
Fosphenytoin + Warfarin
Severity: Major
Fosphenytoin and Warfarin: 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
Phenytoin, the active form of fosphenytoin, interacts with warfarin in a complex fashion. Initial displacement from protein binding and CYP2C9 competition may raise INR, while enzyme induction with ongoing use may lower it. Both bleeding and loss of anticoagulation are possible, and phenytoin levels may rise as well.
What this interaction means in plain words
Fosphenytoin and Warfarin 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; Warfarin belongs to Cardiovascular.
Why it happens
Competitive CYP2C9 metabolism and protein binding displacement, followed by enzyme induction.
What to do
Monitor INR often when starting or stopping fosphenytoin and adjust warfarin accordingly. Monitor phenytoin concentrations too.
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 Warfarin
- Defibrotide + Warfarin Contraindicated
- Ketorolac + Warfarin Contraindicated
- Abciximab + Warfarin Major
- Acetylsalicylic acid + Warfarin Major
- Adagrasib + Warfarin Major
- Amiodarone + Warfarin Major
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
Frequently asked questions
Can I take Fosphenytoin with Warfarin?
MediHelp rates the interaction as major. Fosphenytoin and Warfarin 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. Phenytoin, the active form of fosphenytoin, interacts with warfarin in a complex fashion. Initial displacement from protein binding and CYP2C9 competition may raise INR, while enzyme induction with ongoing use may lower it. Both bleeding and loss of anticoagulation are possible, and phenytoin levels may rise as well.
How serious is the interaction between Fosphenytoin and Warfarin?
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 INR often when starting or stopping fosphenytoin and adjust warfarin accordingly. Monitor phenytoin concentrations too.
What other medicines interact with Warfarin?
MediHelp also lists interactions of Warfarin with, for example, Defibrotide, Ketorolac, Abciximab. The full list is on the Warfarin page.
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