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, largely displaced by DOACs but still prescribed for mechanical heart valves and antiphospholipid syndrome. The narrow therapeutic window is why INR is measured regularly: too little leaves stroke risk, too much drives bleeding. Vitamin K opposes warfarin directly, in supplements and in K2-rich foods like natto. Curcumin at high doses weakly inhibits CYP2C9, the enzyme that clears warfarin's more active S-isomer, though that evidence is mechanism and case-report level. Garlic and ginkgo come up most often, but the mechanism is additive antiplatelet effect rather than a change in warfarin levels: randomised crossover trials found neither altered warfarin pharmacokinetics or INR (Jiang 2005, Mohammed Abdul 2008). The bleeding case reports are still real, and an antiplatelet effect can raise bleeding risk while INR sits unchanged, so both belong with the anticoagulant clinic rather than in an INR reading. St John's Wort induces CYP and drops INR, raising clot risk, and that combination is hard excluded. Cranberry acts on warfarin's own pathway rather than on CYP2C9, and the reported cases cluster at high juice volumes. Standardised cranberry extract has not been tested against warfarin, so it warrants the same monitoring rather than less.
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
Last reviewed 15 June 2026. Each interaction below lists the evidence its assessment drew on: some rest on studies of the exact pair, others on mechanism evidence, where a citation may support one part of the reasoning rather than the exact combination. The evidence line on each entry says which.
CYP induction
Red Does St John's Wort interact with ?
St John's Wort can cause your body to clear warfarin too quickly, dropping your INR and raising clot risk. Do not combine.
Evidence: Supported by human study data.
Amber Does Panax Ginseng interact with ?
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.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Vitamin K pathway
Red Does Vitamin K2 interact with ?
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.
Evidence: Supported by human study data.
Amber Does Coenzyme Q10 interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Green Tea Extract interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Additive anticoagulation
Amber Does Andrographis interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Bromelain interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Cinnamon interact with ?
Whether cinnamon adds to warfarin's effect depends on the type: Cassia cinnamon, the common supermarket kind, contains coumarin, a natural compound that can act like a mild blood thinner at high regular intake, while Ceylon (true) cinnamon contains very little and carries far less 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.
Evidence: Limited human data, based on case reports and a known mechanism.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Cordyceps interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Cranberry Extract (PAC-standardised) interact with ?
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.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Curcumin interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Fisetin interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Ginger interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Glucosamine interact with ?
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.
Evidence: Supported by human study data.
Amber Does L-Carnitine interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Pine Bark Extract interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Quercetin interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Resveratrol interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Spirulina interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Reduced anticoagulant effect
Amber Does Collagen Precursors interact with ?
The 1,000 mg of vitamin C in this collagen precursor blend (a high daily dose) may make warfarin work less well, so your INR (the blood test that shows how thinned your blood is) drops below the level you need. There are reports of large daily doses of vitamin C doing this, with the INR coming 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.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Amber Does Vitamin C interact with ?
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.
Evidence: Supported by human study data.
Reviewer-flagged: awaiting clinical-reviewer sign-off.
Additive antiplatelet effect
Amber Does Garlic Extract interact with ?
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.
Evidence: Supported by human study data.
Amber Does Ginkgo Biloba interact with ?
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.
Evidence: Supported by human study data.
Amber Does Omega-3 EPA interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Amber Does Vitamin E interact with ?
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.
Evidence: Limited human data, based on case reports and a known mechanism.
Nrf2 + CYP modulation
Amber Does Sulforaphane (Broccoli Sprout Extract) interact with ?
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.
Evidence: Based on an established mechanism, with no direct study of this exact combination yet.
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.
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