Documented concern based on cited evidence — not a personalized safety check, not medical advice, and not a decision to combine or avoid. That decision belongs with your prescriber.
Significant interaction — talk to your prescriber before combining
Significant documented interaction. Talk to your prescriber before combining these two.
Mechanism and evidence
Enzyme induction by hyperforin accelerates the metabolism of warfarin, pushing INR down below the protective window. Systematic reviews list warfarin among the drugs whose concentrations or effects fall on St John's wort, driven by the same CYP induction. The effect builds over days of continuous use and outlasts the last dose by one to two weeks.
Evidence tier: review
The tier names the strongest cited study design. Some anchors studied the supplement on its own, or a related drug in the same class — in those pairs the grade rests on that evidence plus established pharmacology, as agreed with our medical reviewer.
What to discuss with your prescriber
- Do not start or stop St John's wort without telling your anticoagulation clinic — INR can drift down with no obvious dietary reason.
- If you have been taking both without the clinic knowing, request an INR check rather than adjusting the warfarin dose yourself.
- After stopping the herb, the induction fades over one to two weeks; keep the clinic informed through that window.