Which supplements raise questions with blood thinners?

The supplements that raise the most questions are those that contain vitamin K, ginkgo, garlic, ginseng, ginger or similar herbs, and enzyme products that affect clotting such as nattokinase. Fish oil also comes up, though the evidence we found is less alarming. Nobody can say from a label whether a product is safe for you, so the answer is to show every product to your prescriber or pharmacist before you start it.

This guide lists what MedlinePlus, NCCIH and published reviews say, why the vitamin K question is different from the others, and how a few supplement labels we audited line up with it. It is a list of questions, not a list of verdicts.

Why do supplements and blood thinners need a conversation?

MedlinePlus explains that blood thinners prevent clots from forming, and that there are two main families. Anticoagulants such as heparin or warfarin slow the body's clot-making process, while antiplatelets such as aspirin and clopidogrel stop platelets from clumping. It says blood thinners may interact with certain foods, medicines, vitamins and alcohol, and that your health care provider must know every medicine and supplement you use. It also notes the main side effect is bleeding, and that you may need regular blood tests to confirm you take enough medicine to prevent clots but not so much that it causes bleeding.

That balance is the whole issue. A product that nudges clotting in either direction can push a carefully tuned dose out of range. A label alone cannot tell your pharmacist how your body will respond, and many supplement labels do not print amounts at all.

A caveat on scope: most of the published interaction research we read concerns warfarin and similar drugs. Newer anticoagulants work differently, so do not assume that a warfarin answer carries over. That is a question for your prescriber.

Which supplements does the research single out?

The most comprehensive source we opened is a 2021 systematic review by Tan and Lee in the British Journal of Clinical Pharmacology. It identified 149 articles describing 78 herbs, foods or dietary supplements that were reported to interact with warfarin. Of those, 45 (57.7 percent) were reported to potentiate warfarin, 23 (29.5 percent) to inhibit it and 10 (12.8 percent) to have limited impact. Twenty herbs and supplements were also reported in connection with bleeding events, from minor bruising and gum bleeding to bleeding inside the skull that led to death. The authors say most foods, herbs and supplements can be taken safely in moderation, and they name a short list of concern that includes Ginkgo biloba, ginger, green tea, chamomile tea, cranberry, cannabis, chitosan and St. John's wort. They advise people to seek professional advice before starting any new herb, food or supplement.

MedlinePlus's drug page for warfarin lists the nonprescription and herbal products that may interact: coenzyme Q10, echinacea, garlic, ginkgo, ginseng, goldenseal and St. John's wort, along with aspirin and NSAIDs such as ibuprofen or naproxen. It tells people not to start any of them without talking to their provider first.

A 2022 review by Hatfield and colleagues, which looked at bleeding, adds names for people on anticoagulants: ginkgo, chondroitin-glucosamine, melatonin, turmeric, bilberry, chamomile, fenugreek, milk thistle and peppermint were associated with bleeding risk in that group. The same review reported that fish oil, ginseng and saw palmetto were not associated with bleeding.

These lists overlap but do not match. That is normal: the evidence is a mix of case reports, small studies and reviews. The consistent advice across them is to ask.

Why is vitamin K different?

Warfarin works against vitamin K, so vitamin K is not an "extra" risk. It is the other side of the same scale. MedlinePlus's warfarin page says some foods and beverages that contain vitamin K can affect how the medicine works, and tells people to eat consistent amounts of vitamin K foods from week to week.

A supplement is exactly that kind of change. In a small 2013 study, Theuwissen and colleagues gave 18 healthy volunteers the vitamin K antagonist acenocoumarol, then added vitamin K2 as MK-7 at 10, 20 and 45 micrograms a day. The authors reported that 45 micrograms lowered the group's average INR, a clotting measure, by about 40 percent, and that doses of 10 and 20 micrograms were judged by two hematologists to cause a clinically relevant INR drop in at least 40 and 60 percent of subjects. They concluded MK-7 supplements need to be avoided in people on this type of drug. The volunteers were healthy and the study was small, so it is not a guide to your own situation, but it shows that even a low dose got a response in some people.

That is why our DentalPrime review notes that the tablet lists vitamin K2 (MK-7), that K2 can interact with warfarin, and that the sales page does not mention it. The review also found no amount stated for K2 on that page. A multivitamin or bone formula with a K2 line item deserves the same question.

What about fish oil, ginkgo and nattokinase?

Ginkgo. NCCIH says ginkgo may increase the risk of bleeding in people taking anticoagulant drugs such as warfarin, and that anyone taking any medicine should talk with a provider before using it. Several offers we audited include ginkgo, so this one comes up often. Our CerebroZen review lists ginkgo among eight named ingredients with no stated amounts. Our Tupi Tea review lists ginkgo with vitamin E and niacin, again with no amounts, and says anyone on prescription medicine should check with a doctor first. In Goliath XL10, the label does print amounts: 130 mg of ginkgo leaf and 130 mg of Asian ginseng, plus 13 mg of vitamin E and 200 mg of hawthorn. Printed amounts help your pharmacist, but they do not answer the interaction question.

Fish oil. The evidence here is milder. A 2016 retrospective study in Nutrients looked at 573 people on long-term warfarin, 145 of whom took fish or krill oil. It found no significant change in time in therapeutic range or in bleeding incidence. It was a retrospective study using records from one pathology clinic, so it does not settle the question for everyone, and the 2022 review above also found no bleeding association for fish oil. Still, tell the clinic you take it. Do not let a reassuring study stand in for the conversation.

Nattokinase and enzyme products. A 2022 clinical paper in Frontiers in Cardiovascular Medicine describes nattokinase as a fibrinolytic and antithrombotic agent. It also mentions a published report of a patient with a mechanical heart valve who developed a clot after using nattokinase in place of warfarin. Take that as a plain warning: a supplement is not a substitute for a prescribed blood thinner, and you should not make that swap without your cardiologist. The Serrasoothe review flags serrapeptase, bromelain and willow bark as ingredients that can affect clotting, and the Tan and Lee review above lists ginger, which is also in that capsule.

What should you do before you buy or start one?

A short routine covers most of it.

1. Make a full list. Prescriptions, over-the-counter pain relievers, vitamins, powders, teas and any new supplement you are considering.
2. Read the label for the high-interest ingredients. Vitamin K, ginkgo, garlic, ginseng, ginger, turmeric, fish oil, vitamin E and enzyme products are worth a flag.
3. Look for amounts. If the label hides amounts inside a blend, your pharmacist cannot judge it. Say so.
4. Ask your prescriber or pharmacist before starting. Do this before the first dose, not after your next lab test.
5. Ask about monitoring. If you do start something with approval, ask whether you need an extra blood test.
6. Do not stop or change your prescription yourself. MedlinePlus tells people to take warfarin exactly as directed and not to stop without talking to the doctor.
7. Report bleeding signs. MedlinePlus lists nosebleeds, bleeding gums, easy bruising and unusual bleeding as reasons to call your doctor.

Our review method explains how we read a label, and the heart health category shows how offers in the area closest to this topic were scored. Our medical disclaimer applies here: we audit offers, not patients.

FAQ

Which supplements interact with blood thinners?

Published sources name vitamin K, ginkgo, garlic, ginseng, ginger, green tea, chamomile, turmeric, St. John's wort and others, with different strengths of evidence. Because the lists differ between sources, your pharmacist should check the exact product against your exact medicine.

Is fish oil safe with warfarin?

One retrospective study of 573 warfarin users found no significant change in warfarin control or bleeding with fish or krill oil, and a 2022 review found no bleeding association. That is reassuring but limited, so tell your prescriber about it and the dose.

Can I take vitamin K2 on warfarin?

Do not start it without asking. In a small study of healthy volunteers on a related drug, MK-7 at 10 micrograms a day was enough to cause a clinically relevant INR drop in some people. Your prescriber may want to check your INR if any vitamin K product is added or removed.

Can I replace my blood thinner with a supplement?

No. A supplement is not a replacement for a prescribed drug, and a published report describes a clot in a patient who used nattokinase in place of warfarin. Never stop a blood thinner without your prescriber's direction.