Is St. John's Wort Safe with warfarin?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Do not take St. John's Wort while on warfarin therapy. It can significantly reduce warfarin's anticoagulant effect, increasing the risk of blood clots or stroke. Inform your anticoagulation clinic if you have been using this supplement.
Why this interaction occurs
St. John's Wort induces CYP enzymes (notably CYP2C9) and P-glycoprotein, accelerating warfarin metabolism and lowering its plasma concentration, thereby reducing anticoagulant effect and increasing thromboembolic risk.
Evidence
Evidence level: strong.
Data source
PubMed - PMID 18637192
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 134,328 reports mentioning warfarin. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the warfarin + supplement combination on this page.
FAERS reports are voluntary and cannot prove that a drug caused an event. Do not use report counts to assess the safety of this specific drug-supplement pair.
Frequently asked questions
Related interactions
All St. John's Wort interactions → · All warfarin interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.