Drug interactions
St john's wort + Warfarin
Severity: Major
St john's wort and Warfarin: 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
St John's wort induces hepatic enzymes and lowers warfarin concentrations, reducing INR and anticoagulant effect. Stopping the herb can cause INR to rise again, leading to bleeding if the dose was increased.
What this interaction means in plain words
St john's wort 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.
Why it happens
Induction of CYP2C9, CYP3A4 and CYP1A2, and possibly P-glycoprotein, by hyperforin increases warfarin clearance.
What to do
Avoid the combination where possible. If the herb is already in use, monitor INR closely and adjust warfarin, rechecking INR after the herb 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?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for St john's wort and Warfarin, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 14 days on the web. No card needed.
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 St john's wort with Warfarin?
MediHelp rates the interaction as major. St john's wort 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. St John's wort induces hepatic enzymes and lowers warfarin concentrations, reducing INR and anticoagulant effect. Stopping the herb can cause INR to rise again, leading to bleeding if the dose was increased.
How serious is the interaction between St john's wort 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: Avoid the combination where possible. If the herb is already in use, monitor INR closely and adjust warfarin, rechecking INR after the herb is stopped.
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.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
