Check Your Supplements Against Warfarin
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Supplements to avoid with warfarin form one of the longest, best-documented interaction lists in medicine. Warfarin has a narrow therapeutic window — a little too much means bleeding, a little too little means clots — and vitamin K, fish oil, ginkgo, St. John’s Wort, cranberry, and high-dose vitamin E can all push it out of range.
The most important supplement interactions with warfarin: vitamin K (directly counteracts the drug — keep intake consistent, including multivitamins and greens powders), St. John’s Wort (speeds up warfarin breakdown, raising clot risk), and bleeding-risk boosters — fish oil, ginkgo biloba, high-dose vitamin E, and large amounts of cranberry. Never add or drop a supplement on warfarin without telling the clinic that monitors your INR.
This page is for informational purposes only and is not medical advice. The interactions listed here are well-documented but not exhaustive — no list can cover every product or personal situation. Always consult your doctor or pharmacist before combining any supplement with prescription medication, and never start or stop a medication or supplement based on what you read online.
Warfarin works by blocking the vitamin K cycle your liver uses to make clotting factors. Its dose is tuned to each patient with regular INR blood tests, and the margin between “working” and “dangerous” is small. Anything that shifts vitamin K intake, adds bleeding risk, or changes how fast your liver clears the drug shows up in your INR.
Supplements can do all three. That is why MedlinePlus tells warfarin patients to check with their doctor before taking any vitamin or herbal product, and to keep their vitamin K intake consistent rather than swinging up and down.
These interactions are documented in standard references. The list is not exhaustive — with warfarin, every new supplement deserves a conversation with your INR clinic first.
| Supplement | Risk | What to do |
|---|---|---|
| Vitamin K (multivitamins, greens powders, K2 supplements) | Directly counteracts warfarin’s mechanism. A sudden increase can drop your INR and raise clot risk; a sudden decrease can raise INR and bleeding risk. | Keep intake consistent. Check multivitamin and greens-powder labels for vitamin K, and report any change to your INR clinic. |
| St. John’s Wort | Induces the enzymes that clear warfarin, lowering drug levels and INR — under-anticoagulation and clot risk. | Avoid entirely. Starting or stopping it both destabilize INR. |
| Fish oil (high-dose omega-3s) | Mild antiplatelet effect; NIH notes high doses may add to bleeding risk with anticoagulants. | Discuss the dose with your doctor; do not change it without telling your INR clinic. |
| Ginkgo biloba | Associated with increased bleeding risk, particularly when combined with anticoagulants. | Avoid while on warfarin. |
| Cranberry (large amounts, concentrated supplements) | Case reports link large amounts of cranberry to elevated INR and bleeding in warfarin patients; MedlinePlus flags cranberry products for warfarin users. | Avoid concentrated cranberry supplements and large amounts of juice; mention any regular use to your clinic. |
| Vitamin E (high doses) | Can inhibit platelet function; NIH cautions that high doses may increase bleeding risk in people on anticoagulants. | Stay near the recommended intake; avoid high-dose vitamin E products. |
Some warfarin patients conclude they must avoid all vitamin K and all supplements forever. That is not what the evidence says. Vitamin K management is about a stable, predictable intake so your dose can be calibrated around it. The genuinely avoid-level items are the herbs that change warfarin metabolism (St. John’s Wort) or stack bleeding risk (ginkgo, high-dose vitamin E).
The operational rule: nothing gets added to or removed from your routine — supplement, herbal tea, greens powder — without your INR clinic knowing. They can almost always accommodate a supplement you care about by monitoring more closely around the change.
A list like this covers the best-documented supplement interactions with warfarin, but it cannot cover every product on the shelf. Multi-ingredient formulas — greens powders, sleep blends, pre-workouts, “hormone support” capsules — often bury a risky ingredient halfway down the label where nobody looks.
Suppi solves this the practical way: scan the label, and the app cross-checks every ingredient against your medications and the rest of your supplement stack using a database built on 500+ peer-reviewed clinical studies. You can also start with our free supplement interaction checker overview to see how it works, then bring your full list to your doctor or pharmacist for the final word.
Sources used for this guide. These are stable, authoritative pages from the U.S. National Institutes of Health, the National Library of Medicine, and the FDA.
This page is for informational purposes only and is not medical advice. The interactions listed here are well-documented but not exhaustive — no list can cover every product or personal situation. Always consult your doctor or pharmacist before combining any supplement with prescription medication, and never start or stop a medication or supplement based on what you read online.
Scan any supplement label and see bleeding-risk and INR-relevant flags in seconds. Free on iOS.
Download Free on iOSPrefer to read first? See how the supplement interaction checker works.