The short answer
St. John's wort can reduce warfarin's anticoagulant effect by increasing its clearance. This is supported by a controlled human study and prescribing information. Starting or stopping the herb can therefore disrupt a warfarin regimen. A few hours of separation has not been shown to prevent this metabolic interaction.
What this answer covers
Oral Hypericum perforatum preparations with oral warfarin. Findings concern systemic repeated exposure; constituent content varies among products and topical use was not studied.
- Supplement or preparation
- St. John's wort
- Medication ingredient
- warfarin
What does the evidence show?
In a crossover study of 12 healthy men, St. John's wort pretreatment increased the apparent clearance of both warfarin enantiomers and reduced the INR-time response after a single warfarin dose. This supports loss of anticoagulant effect rather than impaired tablet absorption. A population-model analysis of overlapping volunteer studies reached a compatible clearance conclusion; it is corroborating analysis, not an independent replication. The warfarin label also lists St. John's wort among products that may decrease its effect.
Which forms and conditions matter?
Extract composition can alter interaction strength, so a result cannot be assigned identically to every Hypericum product. The observed effect followed repeated dosing and reflects metabolism over time. It is not a simple contact interaction in the stomach. The inspected sources do not establish that an unspecified low-hyperforin product, tea or topical preparation behaves the same way as the tested oral extract.
What should I discuss with my care team?
Check with the anticoagulation service before starting St. John’s wort and contact it before stopping an established regimen. Monitoring may need to change as the herb is added or removed. Do not adjust warfarin independently or rely on taking the products a few hours apart. Your clinician can advise on avoiding the combination or managing it with appropriate monitoring.
How certain is the answer?
The direct experiment used healthy men and single-dose warfarin, so it did not measure thrombosis rates during long-term treatment. Nevertheless, reduced exposure and response are directly relevant to warfarin control. The article states the supported direction clearly while leaving exact product magnitude, individual dose changes and follow-up timing to the treating team.
Evidence and practical considerations
Absorption & effectiveness
St. John's wort can reduce warfarin's anticoagulant effect by increasing its clearance. This is supported by a controlled human study and prescribing information. Starting or stopping the herb can therefore disrupt a warfarin regimen. A few hours of separation has not been shown to prevent this metabolic interaction.
- Exact scope
- Oral Hypericum perforatum preparations with oral warfarin. Findings concern systemic repeated exposure; constituent content varies among products and topical use was not studied. with Oral warfarin, including warfarin sodium tablets with the warfarin active moiety.
- Medication form and route
- oral warfarin
- Timing or duration
- Study-specific durations are described in each source exposure; no universally safe exposure duration has been established.
What remains uncertain
- The direct experiment used healthy men and single-dose warfarin, so it did not measure thrombosis rates during long-term treatment. Nevertheless, reduced exposure and response are directly relevant to warfarin control. The article states the supported direction clearly while leaving exact product magnitude, individual dose changes and follow-up timing to the treating team.
Factors that may matter: Exact supplement preparation and other ingredients; Starting, stopping or changing supplement intake; Baseline anticoagulation control and clinician-supervised monitoring.
Read the supporting source summaries
Source 1: current U.S. regulatory prescribing information
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Patients prescribed oral warfarin sodium tablets.
The label advises closer INR monitoring when botanicals are started or stopped, identifies variable botanical composition and lists selected potential interactions. Vitamin K intake should remain consistent.
How this applies: Oral warfarin sodium has warfarin as its active moiety. Label-level botanical and vitamin K monitoring advice applies; it is not proof of this exact supplement effect. Oral Hypericum perforatum preparations with oral warfarin. Findings concern systemic repeated exposure; constituent content varies among products and topical use was not studied.
Regulatory precautions do not quantify the incidence for every exact preparation or prove an interaction for an unlisted ingredient. Warfarin sodium is bridged to the warfarin active moiety only for the oral product.
Source 2: peer-reviewed original research
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: 12 healthy men in a randomized open-label crossover study.
St John's wort increased S- and R-warfarin clearance and reduced the INR-time response. Ginseng did not significantly alter primary pharmacokinetic or pharmacodynamic measures.
How this applies: Use only the Hypericum perforatum or Panax ginseng arm for its exact package. American ginseng and unrelated products are excluded. Exact package scope: Oral Hypericum perforatum preparations with oral warfarin. Findings concern systemic repeated exposure; constituent content varies among products and topical use was not studied.
Single-dose warfarin in healthy men, small sample and selected preparations. Corrected abstract consulted; herb-alone INR findings must not be confused with the warfarin INR-time outcome.
Source 3: peer-reviewed original research
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Pooled data from 24 healthy volunteers previously exposed to single-dose warfarin and selected herbs.
St John's wort substantially increased S-warfarin apparent clearance; Asian ginseng produced a smaller modeled increase. No direct pharmacodynamic herb effect was found.
How this applies: Analysis of overlapping earlier Hypericum perforatum/Panax ginseng participant data. Use only the relevant arm and do not count it as independent clinical replication. Exact package scope: Oral Hypericum perforatum preparations with oral warfarin. Findings concern systemic repeated exposure; constituent content varies among products and topical use was not studied.
Reanalysis of overlapping volunteer data, not an independent clinical trial; no thrombotic or bleeding endpoint and no generalization across ginseng species.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Original evidence and contradictory findings assessed for this exact pair. Article drafted and existing statements adjudicated. Clinical sign-off has not been recorded. | |
| Consumer explanations clarified without changing the evidence assessment. | |
| Research summary published |
Article revision: 7978e914bf662668
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.