Research statusCatalogued record, not clinically approved.
Citation presence and an automated evidence check are not medical review.
This pair has not completed NutriStack's standalone publication gate. Attached sources may mention one or both substances without establishing the stored interaction type, direction, severity, dose, management advice, or combined safety. The page is excluded from search indexing and must not be used to start, stop, or change treatment.
From the databaseWhat the row says.
Every entry follows the same shape: what is happening, the mechanism, and the recommendation.
Scope
Supplement × Prescription
Last updated
July 13, 2026
ContraindicatedStrong evidence
What is happening. St. John's Wort is a potent inducer of CYP3A4, CYP2C9, and P-glycoprotein, which dramatically accelerates warfarin metabolism and reduces its plasma levels. This can cause a clinically significant drop in INR, leading to loss of anticoagulant protection and potential thromboembolic events including stroke.
Mechanism. Hyperforin in St. John's Wort is a potent activator of the pregnane X receptor (PXR), which upregulates CYP3A4, CYP2C9, and P-glycoprotein expression. Both S-warfarin (CYP2C9 substrate) and R-warfarin (CYP3A4 substrate) are metabolized faster, reducing anticoagulant effect by up to 50%.
Recommendation. Do not take St. John's Wort with warfarin. This is a well-established dangerous interaction. If you are currently taking both, consult your prescriber immediately, do not abruptly stop St. John's Wort as INR may rebound sharply.
Direction. St. John's Wort changes how Warfarin behaves.
Stack Score
How it moves the number.
Effect on the composite score
No numeric score interpretation is published for this unreviewed pair record.
Stack Score is a NutriStack product heuristic, not a clinical risk or benefit grade. An unreviewed database label must not be treated as a safety or benefit signal. The algorithm is documented at /methodology/stack-score.
SourcesSources, by evidence tier.
Sources attached to the record. Some support an ingredient or mechanism rather than a study of the exact pair; PMIDs link to PubMed where available.
FAQCommon St. John's Wort + Warfarin questions.
Plain-language answers drawn from the interaction record on this page.
Can I take St. John's Wort and Warfarin together?
The NutriStack record classifies St. John's Wort and Warfarin as contraindicated. Confirm the combination urgently with a clinician or pharmacist and do not stop prescribed medicine abruptly. St. John's Wort is a potent inducer of CYP3A4, CYP2C9, and P-glycoprotein, which dramatically accelerates warfarin metabolism and reduces its plasma levels. This can cause a clinically significant drop in INR, leading to loss of anticoagulant protection and potential thromboembolic events including stroke.
What kind of interaction is it?
Contraindicated: the stored record says the combination should be avoided; confirm urgent or medication-specific action with a clinician or pharmacist.
How does this affect a NutriStack Stack Score?
No numeric score interpretation is published for this pair because its stored label has not completed source-level clinical review.
What do the cited sources establish?
NutriStack lists 4 source records with this St. John's Wort + Warfarin entry. A source may support an ingredient, mechanism, or safety concern without testing the exact pair, so citation presence alone does not prove the visible verdict. Full sourcing methodology is at https://nutristackapp.com/methodology/interactions.
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