Drug Interaction Report

Warfarin and Heparin: Interaction Details

A pharmacist-reviewed analysis of the Warfarin and Heparin interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Warfarin

Coumadin Jantoven
+

Heparin

No brand names on record
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
How we grade severity & evidence

Severity levels

  • Contraindicated: These should generally not be used together.
  • Major: Potentially serious — often needs a change or close monitoring.
  • Moderate: Can be significant — usually manageable with monitoring.
  • Minor: Usually limited clinical impact.

Evidence grades

  • Established: Well documented — supported by controlled studies or strong clinical data.
  • Probable: Good supporting evidence, though not definitively proven.
  • Suspected: Some evidence suggests this interaction, but it is not well established.
  • Possible: Limited or conflicting evidence; the interaction may occur.
  • Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.

Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.

Of 210 documented Warfarin interactions, 160 are rated major — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
unspecified
Evidence
probable
Severity
Major

What happens

An increased risk of bleeding

Interaction Deep Dive

The concomitant use of warfarin with other anticoagulants, such as heparin, may increase the risk of bleeding due to additive anticoagulation2. When converting from heparin therapy to oral warfarin therapy, it is desirable to overlap both treatments for 4 to 5 days with full-dose heparin therapy until warfarin has achieved the desired prothrombin time (PT)/INR. Heparin therapy may then be discontinued without tapering. Heparin prolongs one-stage PT; however, during initial warfarin therapy, the clinical significance of the interaction is minimal. When concomitant use is required, more frequent monitoring of PT/INR is recommended3. When measuring prothrombin time, unless using continuous heparin IV infusion, wait at least 5 hours after the last IV bolus heparin dose (or 24 hours after the last subQ dose) to ensure a valid reading 1.

Why it happens (mechanism)

Additive anticoagulation

Common questions

Can I take Warfarin and Heparin together?

An increased risk of bleeding Always confirm with your pharmacist or prescriber before making any change.

How serious is the Warfarin and Heparin interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "unspecified". The timing of this interaction is not well characterized.

How strong is the evidence for this interaction?

The evidence is graded "probable". Good supporting evidence, though not definitively proven.

From our Q&A

Real reader questions about these medications, each personally answered by our pharmacist:

Questions for your pharmacist

  • Does my dose of Warfarin or Heparin need adjusting while I take them together?
  • What symptoms should prompt me to call you or my prescriber right away?
  • Does the timing of my doses matter for this combination?
  • Is there a safer alternative to one of these medications for me?

References (3)

  1. Product Information: heparin sodium IV, subcutaneous injection, heparin sodium IV, subcutaneous injection. APP Pharmaceuticals, LLC, Schaumburg, IL, 2010.
  2. Product Information: WARFARIN SODIUM oral tablets, warfarin sodium oral tablets. Teva Pharmaceuticals (per Dailymed), Parsippany, NJ, 2023. DailyMed
  3. Product Information: COUMADIN(R) oral tablets, intravenous injection, warfarin sodium oral tablets, intravenous injection. Bristol-Myers Squibb Company, Princeton, NJ, 2010. DailyMed
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Warfarin

Keep reading about Warfarin

All Warfarin interactions (210)
Also serious with: Defibrotide Injection Penicillin V Potassium Cefixime Cyclophosphamide
Heparin

Keep reading about Heparin

All Heparin interactions (221) Heparin drug information
Also serious with: Telavancin Oritavancin Defibrotide Defibrotide Injection
Beyond drug–drug

These medications also interact with supplements

Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:

Warfarin + Activated Charcoal Alfalfa American Ginseng Beer Cannabis Danshen Dong Quai Fasting + 353 more
Heparin + Danshen Piracetam Aconite Allspice Aloe Alpha-lipoic Acid Andrographis Arnica + 194 more

major · moderate · minor — check everything you take with our drug–supplement interaction checker.

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.