Drug & supplement interaction

Fasting + Warfarin

Warfarin brand name: Warfarin.

Major Interaction
Medically reviewed by · Updated September 2026
The bottom line Major

Warfarin falls into 2 drug categories that Fasting can interact with; the most serious is rated major. Theoretically, fasting may increase the risk of bleeding if used with warfarin.

Read the full interaction report
SeverityMajor
LikelihoodProbable
Interactions2
EvidenceD
What do Severity, Likelihood & Evidence mean?
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Interaction report

Fasting and Warfarin

These two interact in 2 distinct ways. Each one is explained separately below.

Why Are There 2 Interactions?

Fasting interacts with whole categories of medication. Warfarin falls into 2 of those categories, so it can interact with Fasting in 2 different ways. We’ve listed each one below — your overall risk is driven by the most serious of them (Major).

Major

WARFARIN (Coumadin)

1 of 2
Theoretically, fasting may increase the risk of bleeding if used with warfarin.
Both postoperative fasting and Ramadan fasting have been found to increase the INR in patients previously stabilized on warfarin. Closely monitor patients who are fasting while taking warfarin. Dose adjustment may be necessary.
Severity Major Likelihood Probable Evidence D

Interaction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports..

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's something a lot of folks don't realize about warfarin: changes in how and what you eat can nudge your numbers around. When you fast (whether it's before surgery or during Ramadan), reports have shown the INR can creep up, meaning your blood thins out a little more than usual. That can raise the chance of bleeding, like easy bruising, nosebleeds, bleeding gums, or in more serious cases blood in your urine or stool.

Warfarin is a tricky, narrow-margin medicine, so even small shifts matter. This warning leans more on real-world reports than hard studies, so don't panic. But please don't start a fasting routine without telling your doctor or pharmacist first. They may want to check your INR more often and adjust your dose to keep you in a safe range.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Katada Y, Nakagawa S, Nishimura A, et al. Effects of fasting on warfarin sensitivity index in patients undergoing cardiovascular surgery. Eur J Clin Pharmacol. 2019;75(4):561-568. PubMed
  2. Lai YF, Cheen MHH, Lim SH, et al. The effects of fasting in Muslim patients taking warfarin. J Thromb Haemost. 2014;12(3):349-54. PubMed
Moderate

ANTICOAGULANT/ANTIPLATELET DRUGS

2 of 2

Why this applies to Warfarin

Warfarin falls under ANTICOAGULANT/ANTIPLATELET DRUGS — the group Fasting interacts with.

Fasting may increase the risk of bleeding if used with anticoagulant or antiplatelet drugs.
Fasting has been found to increase the international normalized ratio (INR) in patients stabilized on warfarin. Closely monitor patients who are fasting while taking anticoagulant or antiplatelet drugs. Dose adjustment may be necessary.
Severity Moderate Likelihood Possible Evidence D

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.

Dr. Brian Staiger, PharmD, BCPS
Dr. Brian Staiger explains PharmD, Clinical Pharmacist

Here's the thing with warfarin: it's a blood thinner with a very narrow margin, so even small shifts can tip you out of your safe zone. Your INR is the lab number that tells us how thin your blood is. When folks fast, their eating patterns change, including how much vitamin K (from leafy greens) they take in, and the body's metabolism shifts too. We've seen reports where fasting nudged the INR higher in people who were stable on warfarin.

A higher INR means more bleeding risk, think easy bruising, nosebleeds, bleeding gums, or blood in urine or stool. This is based mostly on anecdotal reports, so it's not a sure thing, but with warfarin I'd never wing it. Please talk with your doctor or pharmacist before fasting so they can check your INR more closely. Don't change your warfarin on your own.

Contact Dr. Brian directly if you have additional questions on this interaction
References
  1. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One. 2019;14(1):e0209353. PubMed
  2. Katada Y, Nakagawa S, Nishimura A, et al. Effects of fasting on warfarin sensitivity index in patients undergoing cardiovascular surgery. Eur J Clin Pharmacol. 2019;75(4):561-568. PubMed
  3. Lai YF, Cheen MHH, Lim SH, et al. The effects of fasting in Muslim patients taking warfarin. J Thromb Haemost. 2014;12(3):349-54. PubMed

What These Ratings Mean

Every interaction above is graded three ways — here’s the key to each label.

Severity how clinically significant the interaction is
Major

Clinically significant. Avoid the combination, or use only under close professional supervision.

Moderate

Usually worth avoiding or managing — may need monitoring, a dose change, or timing apart.

Minor

Generally not serious. Be aware of it and mention it to your pharmacist.

Likelihood how strong the evidence is that the interaction actually happens in people
  • LikelyWell-controlled human studies have demonstrated the likely existence of this interaction.
  • ProbableInteraction has not been documented in well-controlled studies, however, the interaction has been demonstrated in some small human studies or in controlled animal studies in conjunction with multiple case reports.
  • PossibleInteraction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists.
  • UnlikelyInteraction has been demonstrated in animal or in lab research but has been shown not to occur in humans.
Evidence the quality of the studies behind the rating (graded A–D)
  • AHigh-quality meta-analysis (quantitative systematic review).
  • BCase-control study.
  • CConsensus.
  • DAnecdotal evidence.

These ratings are for information only. Never start, stop, or otherwise change your therapy before speaking with your provider.

Keep it in perspective

Fasting Also Interacts With Other Medications

Fasting has 1 major, 206 moderate known drug interactions in our database — the Warfarin one above is among them. If you take anything else, check it against the full list below.

Major 1 Moderate 206 No known interaction 2,623 (incl. Warfarin)
Taking anything else?

Check Another of Your Medications Against Fasting

Covers the 2,830 medications we’ve checked against Fasting. Pick one to open its full report.

From our Q&A

Questions About Fasting or Warfarin

These are real questions from readers, answered by the pharmacists at HelloPharmacist — written by a person, not generated from a database. Each one covers Fasting or Warfarin, not necessarily both, but if one is close to what you were about to ask it will usually go further than a database entry can:

Not quite your question? Ask our pharmacists directly — it is free, there is no account required, and answers usually come back within 24 hours.

Everything else you take

See Every Supplement That Interacts With Warfarin

This page covers Fasting only. If you take other supplements — a multivitamin, a sleep aid, something for joints — the Warfarin page lists 361 supplements it has a documented interaction with, worst first, each one explained.

The most serious there right now: Activated Charcoal (major), Alfalfa (major), American Ginseng (major), Beer (major) and Cannabis (major).

By severity: 16 major · 301 moderate · 44 minor.

From our pharmacist’s overview on the Warfarin page

“People take warfarin to prevent dangerous clots, and many are the same people reaching for glucosamine for sore joints or a multivitamin containing vitamin K, which makes this a list that deserves genuine respect. Of the 361 supplements on file for warfarin, 16 carry a major rating, and the danger runs in both directions: some add to w…”

Reviewed by 2 · Jul 18, 2026

Open the Warfarin interaction page

How much people look this up

Where Fasting and Warfarin Sit Against Everything Else

Out of every supplement and medication people look up on HelloPharmacist, here is where these two land this week — and which way each has been moving.

Supplement Fasting
Top half

#685 of 1,452 supplements looked up here in the last 7 days

29 days ago today
Medication Warfarin
Top 10

#4 of 2,830 medications looked up here in the last 7 days

29 days ago today

Position among everything looked up on this site, not a measure of how commonly either one is used or how important it is. A supplement can climb this list because of a news story. It counts every request to the page, automated ones included, and nothing here identifies anyone.

Eating pattern

About Fasting

What is Fasting?

  • Weight loss
  • Blood sugar control
  • Heart health
  • Religious or spiritual practice
  • General wellness and longevity

Fasting means voluntarily going without food (and sometimes drink) for set periods. Some research suggests it may help with weight loss and blood sugar in certain people, but results vary and quality of evidence is mixed. It is not safe for everyone, so check with your doctor or pharmacist before starting—especially if you take medication or have a health condition.

Read the full Fasting monograph & all interactions
Medication

About Warfarin

What Is Warfarin Used For?

Brand name: Warfarin

Warfarin sodium tablets and Jantoven tablets are both used to prevent and treat venous thrombosis (blood clots in the veins) and pulmonary embolism (a clot in the lungs). They are also used to prevent clot-related complications in people with atrial fibrillation (an irregular heart rhythm) or who have had a heart valve replaced.

In people who have had a heart attack, warfarin is used to reduce the risk of death, another heart attack, stroke, or other clotting events. It does not dissolve an existing clot, but it helps prevent the clot from growing and stops new dangerous clots from forming.

See full Warfarin drug information
Keep checking

Other Drugs & Fasting

Browse every other medication we’ve checked against Fasting — both the ones with a known interaction and the ones without. Select any drug for the full detail.

1Major
206Moderate
0Minor
2,622No known interaction
1 Major 206 Moderate 2,622 No interaction
Major

1 interaction

Moderate

206 interactions

Showing 60 of 206 moderate interactions — see all 206 on the Fasting page.

No interaction 2,622 drugs checked — show the list

We also checked Fasting against these medications and found no known interaction in our sources. Open any page to confirm.

Showing the 60 most looked-up of 2,622 medications checked with no known interaction — see the full Fasting list.

See the full Fasting interactions page
See every supplement checked against Warfarin, ranked by severity
Sources

Sources & How We Checked

Interaction details are evidence-graded and sourced from the Natural Medicines database. The medication overview comes from MedlinePlus.

Content is written and reviewed by licensed HelloPharmacist pharmacists. See our data sources and editorial standards for how this information is built and checked.

Parts of this content are provided by the Therapeutic Research Center, LLC.

DISCLAIMER: Currently this does not check for drug-drug interactions. This is not an all-inclusive comprehensive list of potential interactions and is for informational purposes only. Not all interactions are known or well-reported in the scientific literature, and new interactions are continually being reported. Input is needed from a qualified healthcare provider including a pharmacist before starting any therapy. Application of clinical judgment is necessary.

© 2021 Therapeutic Research Center, LLC

Keep exploring
Fasting monograph › Uses, safety, and every drug interaction in one place Warfarin supplement interactions › Every supplement checked against Warfarin, ranked by severity Warfarin drug information › The full plain-English guide to this medication Supplement interaction checker › Check any supplement or product against your medications Browse medications A–Z › Every drug we've checked against herbs & supplements Ask a pharmacist › Free, personal answer from our licensed team
Pharmacist Counseling Corner

Following Fasting While Taking Warfarin: Common Questions

Can you follow Fasting while taking Warfarin?
This is rated a major interaction, so the combination is best avoided unless your doctor or pharmacist has specifically told you otherwise. Read the full details on this page and confirm with your pharmacist before combining them.
Why are there 2 interactions listed for Fasting and Warfarin?
Warfarin belongs to more than one drug category, and Fasting interacts with each of those categories a little differently. We list every one so nothing is missed — your overall risk is driven by the most serious of them.
How serious is the Fasting and Warfarin interaction?
We rate it Major. This is a clinically significant combination that is generally best avoided, or used only under the direct supervision of your physician or pharmacist.

Written and reviewed by the HelloPharmacist editorial staff. Our editorial policy

Still Have a Question About Warfarin and Fasting?

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This information is for education only and is not a substitute for professional medical advice. Always check with your pharmacist or doctor before starting, stopping, or combining supplements and medications.