Supplements and Warfarin sodium.
Warfarin sodium, sold under the brand names Coumadin, Jantoven, is a vitamin-K antagonist anticoagulant. It sits in a narrow therapeutic window monitored by INR.
Warfarin is the original vitamin K antagonist anticoagulant. It is gradually being replaced by DOACs (apixaban, rivaroxaban, edoxaban) in atrial fibrillation, but still in active UK prescribing for mechanical heart valves, antiphospholipid syndrome, and where DOAC contraindications apply. The defining feature is the narrow therapeutic window. INR is measured regularly because under-treatment leaves stroke risk on the table, while excess anticoagulation drives bleeding. The supplement surface is one of the largest in clinical pharmacology. Vitamin K opposes warfarin directly, whether in supplements or in foods rich in K2 like natto. CYP2C9 inhibitors (curcumin at high doses, garlic, and ginkgo in some studies) raise warfarin plasma levels and push INR up. CYP induction by St John's Wort drops INR and raises clot risk. This combination is hard excluded. Cranberry juice has been flagged historically, though the pooled trial evidence is mixed. Anyone on warfarin considering a new supplement should ask their anticoagulant clinic before starting.
Below are the 26 documented pairs we have explicitly assessed against Warfarin sodium in the Distil database: 2 red and 24 amber. The pairs cluster around 6 mechanisms: CYP induction, Vitamin K pathway, Additive anticoagulation, Reduced anticoagulant effect, Additive antiplatelet effect, and Nrf2 + CYP modulation. Every call is cited to either a clinical reference (PMID) or the British National Formulary. Anything not on this list is either still to be assessed or beyond our database scope. The checker beneath surfaces assessments by supplement, and the missing-item form at the bottom of the page routes any uncatalogued supplement into our next curation pass.
Documented interactions
CYP induction
St John's Wort can cause your body to clear warfarin too quickly, dropping your INR and raising clot risk. Do not combine.
Ginseng tends to work against warfarin rather than with it. Some people on warfarin see their INR drop when they add ginseng, which can leave them under-protected against clots. The evidence is mixed between ginseng types, so if you take warfarin, tell your anticoagulation clinic before starting ginseng and ask for an INR check a couple of weeks in.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Vitamin K pathway
Vitamin K is the nutrient warfarin works against. Supplementing vitamin K can blunt warfarin's effect and raise clot risk. Do not start, stop, or change vitamin K supplements without your anticoagulation clinic's guidance.
CoQ10 has a similar molecular shape to vitamin K and may reduce warfarin's effect for some people. If you start or stop CoQ10 on warfarin, check INR within two weeks.
Green tea can work against warfarin, but the documented cases involve drinking large volumes of brewed green tea, which carries vitamin K. Concentrated green tea extract capsules contain far less vitamin K, so the same effect is less likely from a standard supplement. Either way, keep your green tea habit steady rather than changing it suddenly while on warfarin, and tell your anticoagulation clinic what you take.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Additive anticoagulation
Andrographis may raise warfarin's effect, both by mild blood-thinning of its own and by slowing how the body clears warfarin. The direct warfarin evidence so far is from animal studies rather than people. If you take warfarin, tell your anticoagulation clinic before starting andrographis and ask for an INR check a couple of weeks in.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Bromelain has a mild blood-thinning effect of its own and may add to warfarin's. If you take warfarin, tell whoever monitors your INR before adding bromelain and check your INR if you do start it. If you have surgery planned, mention bromelain to your team; stopping high-dose bromelain about two weeks beforehand is a common precaution.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
This depends on the type of cinnamon. Cassia cinnamon, the common supermarket kind, contains coumarin, a natural compound that can act like a mild blood thinner at high regular intake and may add to warfarin's effect. Ceylon (true) cinnamon contains very little coumarin and carries far less of this concern. A pinch on food is not the issue; the concern is large daily doses or concentrated cinnamon supplements. If you take warfarin and use cinnamon regularly in large amounts, tell your GP and check your INR.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Cordyceps may add to warfarin's blood-thinning effect through a mild antiplatelet action of its own. This is based on laboratory work on human platelets rather than studies in people taking warfarin, so the real-world size of the effect is unknown. If you take warfarin, mention cordyceps to your anticoagulation clinic and ask whether an INR check is worth doing after you start.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Cranberry can occasionally push warfarin's blood-thinning effect higher, mainly when taken as large volumes of cranberry juice. Standardised cranberry extract at studied doses appears safer than high-volume juice, but tell your GP either way and ask whether INR monitoring should be a little tighter for the first few weeks.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Curcumin may add to warfarin's blood-thinning effect. If you take warfarin, ask your GP before adding curcumin and monitor your INR closely if you do.
Fisetin may mildly thin the blood by reducing how platelets clump, which could add to warfarin and nudge bleeding risk up. The concern is with a high-dose fisetin course, not the small amount in food. If you take warfarin, tell your anticoagulant clinic before starting a fisetin course and expect closer INR checks at first.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Ginger has a reputation for thinning the blood, but the controlled studies are reassuring. When ginger was given to people taking warfarin in proper trials, it did not change their INR or how warfarin behaved. A small number of case reports describe raised INR or bleeding, so the safe approach is to keep your ginger intake steady rather than suddenly taking large supplement doses, and to mention it to whoever monitors your INR. There is no need to avoid normal dietary ginger.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Glucosamine can push warfarin's blood-thinning effect higher, raising your INR and your bleeding risk. This shows up more when the glucosamine dose goes up. If you take warfarin, tell your anticoagulation clinic before starting glucosamine and ask for an INR check within a couple of weeks, especially if you change the dose.
There is a single report of L-carnitine strengthening the effect of a warfarin-type blood thinner, raising the risk of bleeding. The report involved a closely related drug rather than warfarin itself, so the evidence is thin, but the drugs work the same way. If you take warfarin, mention L-carnitine to your anticoagulation clinic and ask whether an INR check is worth doing after you start.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Pine bark extract (pycnogenol) has a mild blood-thinning effect of its own, shown in people as reduced platelet stickiness. It has not been studied alongside warfarin, so the size of any combined effect is unknown. If you take warfarin, tell your anticoagulation clinic before starting pine bark extract and ask for an INR check a couple of weeks in.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Quercetin may, in theory, add to warfarin's blood-thinning effect at high doses, though a small human study found no change in INR. If you take warfarin, mention any high-dose quercetin to your GP and monitor your INR if you start it.
Resveratrol can act on warfarin in two ways: it can slow how the body clears warfarin and it can mildly thin the blood by reducing how platelets clump. Both could nudge your INR up and raise bleeding risk. The concern is with supplement doses, not the small amount in food. If you take warfarin, tell your anticoagulant clinic before starting resveratrol and expect closer INR checks at first.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Spirulina has a mild antiplatelet signal in laboratory studies, but a human trial at a high dose found no change in clotting tests or platelet activity. Any effect on warfarin is likely to be small. If you take warfarin, keep your spirulina intake steady rather than starting or stopping suddenly, and tell your anticoagulation clinic what you take.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Reduced anticoagulant effect
This collagen precursor blend contains 1,000 mg of vitamin C, which is a high daily dose. There are reports of large daily doses of vitamin C making warfarin work less well, so your INR (the blood test that shows how thinned your blood is) drops below the level you need. In each case the INR came back up once the vitamin C was stopped. If you take warfarin, tell whoever manages it before starting or stopping this blend, so your INR can be watched and kept stable.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
There are reports of large daily doses of vitamin C making warfarin work less well, so your INR (the blood test that shows how thinned your blood is) drops below the level you need. In each case the INR came back up once the vitamin C was stopped. A normal dietary amount or a small daily supplement is not the concern here. If you take a high-dose vitamin C supplement, tell whoever manages your warfarin so your INR can be watched, and avoid starting or stopping it without letting them know.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Additive antiplatelet effect
Garlic at supplement doses has a mild blood-thinning effect of its own. Combined with warfarin, that can shift INR. If you take both, mention the garlic supplement to whoever monitors your INR and do not stop or start without telling them.
Ginkgo biloba has a mild antiplatelet effect of its own. The clinical data on whether it meaningfully shifts INR is mixed, but bleeding events have been reported, so combining with warfarin is best done with INR monitoring rather than freely.
At supplement doses below 1 gram per day, omega-3 fish oil has minimal effect on INR. At higher doses (3 grams per day and above) used for triglyceride lowering, the antiplatelet effect is more meaningful and INR can shift. Tell your GP what dose you take if you are on warfarin.
Vitamin E at low supplement doses (100 to 200 IU) has little effect on warfarin. At high doses (above 400 IU per day) it can add to bleeding risk and may nudge your INR. Tell your anticoagulant clinic if you take high-dose vitamin E, and stop it six weeks before planned surgery.
Nrf2 + CYP modulation
Sulforaphane affects liver enzymes responsible for clearing warfarin (mainly CYP2C9). The direction and magnitude in humans at supplement doses are not well characterised. Tell your GP and ask whether INR monitoring should be tightened for the first few weeks.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
What this list does not say. Pairs not flagged here are not implicitly safe. They are either not yet in our database, or fall outside our inclusion scope (food-supplement interactions only; for drug-drug interactions, the BNF is authoritative). Use the checker below to surface any supplement, and submit a missing item if you take something we have not catalogued.
How we grade severity, choose what's in scope, and what we exclude.
Every call on this page is reasoned. We publish the full rubric for severity tiers, the medication inclusion logic, the evidence grades we accept, and what we deliberately leave out. About three thousand words. Worth reading once if you use this tool more than occasionally.
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