Drug interactions
Nirmatrelvir / ritonavir + Warfarin
Severity: Moderate
Nirmatrelvir / ritonavir and Warfarin: 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
Ritonavir affects several enzymes involved in warfarin metabolism, and the INR may rise or fall during and after the short course. Infection itself and reduced oral intake can also destabilize the INR.
What this interaction means in plain words
Nirmatrelvir / ritonavir and Warfarin 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
Ritonavir inhibits CYP3A4 and may variably affect CYP2C9 and other enzymes, producing unpredictable changes in warfarin clearance.
What to do
Check INR during treatment and again after completing the course, and adjust the warfarin dose based on results.
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 Warfarin
- Defibrotide + Warfarin Contraindicated
- Ketorolac + Warfarin Contraindicated
- Abciximab + Warfarin Major
- Acetylsalicylic acid + Warfarin Major
- Adagrasib + Warfarin Major
- Amiodarone + Warfarin Major
Frequently asked questions
Can I take Nirmatrelvir / ritonavir with Warfarin?
MediHelp rates the interaction as moderate. Nirmatrelvir / ritonavir and Warfarin 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. Ritonavir affects several enzymes involved in warfarin metabolism, and the INR may rise or fall during and after the short course. Infection itself and reduced oral intake can also destabilize the INR.
How serious is the interaction between Nirmatrelvir / ritonavir and Warfarin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Check INR during treatment and again after completing the course, and adjust the warfarin dose based on results.
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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