What should you not take with vitamin K?

January 14, 2026 •

4 min read

Vitamin K is essential for producing four of the 13 blood-clotting factors, a role that can be dangerously interfered with by certain medications and supplements. While vital for bodily functions like blood clotting and bone health, managing vitamin K intake is crucial for those on certain medications.

Quick Summary

Crucial interactions exist between vitamin K and specific medications, including blood thinners like warfarin, and various supplements. Maintaining a consistent vitamin K intake is paramount for those on anticoagulants, while high doses of certain vitamins and herbs should be avoided due to interference.

Key Points

Anticoagulant Alert: If you take blood thinners like warfarin, your primary concern is maintaining a consistent, stable intake of vitamin K from food and supplements to avoid dangerous fluctuations in your medication's effectiveness.
Supplements to Avoid or Use Cautiously: High-dose vitamin E supplements, as well as certain herbs like ginkgo biloba, garlic supplements, St. John's Wort, and ginseng, can interfere with blood clotting and should be avoided or used with caution.
Medication Interference: Certain drugs beyond anticoagulants, including some antibiotics (especially cephalosporins), cholesterol-lowering bile acid sequestrants, and the weight-loss drug orlistat, can interfere with vitamin K production or absorption.
Consistency Over Avoidance: For those on warfarin, the goal is not to eliminate vitamin K-rich foods but to consume them consistently. Sudden, drastic changes in diet are what pose the greatest risk.
Talk to Your Doctor: Never start, stop, or significantly alter your vitamin or supplement intake without first consulting your healthcare provider, particularly if you are on any medication.
Be Mindful of Combination Products: Read labels carefully on multivitamins, nutritional shakes, and herbal remedies, as they may contain ingredients that can affect your vitamin K levels.

The Most Critical Interaction: Anticoagulant Medications

The most significant and potentially dangerous interaction for individuals concerns vitamin K and anticoagulant drugs, such as warfarin (Coumadin). Warfarin works by antagonizing, or counteracting, the effects of vitamin K, which is a cofactor in the synthesis of blood-clotting proteins. This delicate balance is carefully calibrated by a doctor to ensure the medication thins the blood just enough to prevent dangerous clots without causing excessive bleeding.

For this reason, the key for anyone on warfarin is not to avoid vitamin K, but to maintain a consistent, steady intake from week to week.

The Danger of Fluctuating Vitamin K Intake

Sudden Increase in Vitamin K: Consuming a large, inconsistent amount of vitamin K-rich foods or supplements can make warfarin less effective. This reduces the anticoagulant's ability to prevent clots, increasing the risk of a dangerous thrombosis.
Sudden Decrease in Vitamin K: Conversely, an abrupt drop in vitamin K intake can increase warfarin's effect, putting a person at a higher risk of serious bleeding. This could happen, for instance, if a person on a vitamin K-rich diet becomes ill and is unable to eat for several days.

Key Foods to Monitor

Patients on warfarin are not required to eliminate vitamin K from their diet, but they must be mindful of consistent consumption of certain high-vitamin K foods. It is wise to discuss dietary habits with a healthcare provider to determine an appropriate, consistent weekly intake. Some of the highest sources of vitamin K include:

Dark Leafy Greens: Kale, spinach, collard greens, mustard greens, and turnip greens.
Cruciferous Vegetables: Broccoli, Brussels sprouts, cabbage, and asparagus.
Other Foods: Soybeans, edamame, and beef liver.

Supplements That Can Interfere with Vitamin K

Beyond diet, several dietary and herbal supplements can impact the body's vitamin K levels or its clotting mechanisms, particularly for those on anticoagulants. Consulting a doctor before starting any new supplement is essential.

Other Vitamins

High-Dose Vitamin E: Supplementing with high doses of vitamin E (over 800 IU) can interfere with vitamin K-dependent blood clotting and may increase the risk of bleeding. This is especially important for those taking blood-thinning medication.
Vitamin D: Some research suggests a complex relationship, with high doses of vitamin D possibly promoting the production of vitamin K-dependent proteins that may remain inactive if vitamin K levels are insufficient. Taking optimal, balanced amounts of both vitamins is recommended for bone and cardiovascular health, but high-dose supplementation should be supervised.

Herbal and Dietary Supplements

Ginkgo Biloba: This popular herb can have blood-thinning effects and should be avoided by people on warfarin, as it may increase bleeding risk.
Garlic (Supplements): While culinary garlic is generally safe, concentrated garlic supplements can increase the risk of bleeding, especially when combined with anticoagulants.
St. John's Wort: Used for depression, this herb is known to reduce the effectiveness of warfarin.
Cranberry Products: Large quantities of cranberry juice or capsules can increase the effect of warfarin and raise the risk of bleeding.
Ginseng: This supplement may decrease the effectiveness of warfarin.
Coenzyme Q10 (CoQ10): High-dose CoQ10 is structurally similar to vitamin K and may require a warfarin dose adjustment, as it can decrease the medication's effect.

Prescription Medications to Watch

In addition to anticoagulants, other prescription medications can impact vitamin K levels or interfere with its absorption.

Antibiotics

Prolonged use of certain antibiotics, particularly cephalosporins, can disrupt the gut bacteria that produce vitamin K. This can lead to decreased vitamin K levels, especially if a person also has a poor vitamin K intake, potentially increasing bleeding risk.

Cholesterol-Lowering Drugs

Bile acid sequestrants (e.g., cholestyramine, colestipol) are used to lower cholesterol. They can, however, reduce the absorption of fat-soluble vitamins like vitamin K because they bind to bile acids that are essential for fat absorption.

Weight-Loss Drugs

Orlistat (Alli, Xenical) is a medication that reduces the amount of dietary fat the body absorbs. Since vitamin K is a fat-soluble vitamin, this effect can also lead to a decrease in vitamin K absorption.

Food vs. Supplement Vitamin K on Warfarin


Aspect	Consistent Vitamin K Intake (Food)	Inconsistent Vitamin K Intake (Supplement/Food)
Effect on INR	Allows stable INR (International Normalized Ratio), making warfarin dosage predictable.	Causes unpredictable INR fluctuations, potentially leading to dangerous bleeding or clotting.
Risks	Lowers the risk of clotting or bleeding due to stable warfarin efficacy.	Significantly increases the risk of bleeding or thromboembolic events.
Management	Part of a stable, balanced diet, regularly monitored by a doctor.	Creates a need for frequent dosage adjustments and blood tests.
Doctor's Advice	Most often endorsed, as it supports long-term medication stability.	Usually discouraged unless under strict medical supervision.

A Critical Conclusion for Supplement Users

While vitamin K is a vital nutrient for most people, its interaction with certain drugs and other supplements makes informed decision-making paramount. The most crucial takeaway is for individuals on anticoagulant therapy like warfarin. Instead of avoiding vitamin K, they must maintain a consistent dietary intake to ensure their medication works predictably and safely. For everyone else, high doses of supplements like vitamin E and certain herbs like ginkgo biloba should be approached with caution due to their potential to interfere with blood clotting. Always discuss any dietary changes, new supplements, or over-the-counter medications with a healthcare provider, especially if you are managing a chronic condition or taking prescription drugs. Your doctor is the best resource for navigating these complex interactions and ensuring your health and safety. You can also consult authoritative sources for more detailed information.

Do You Have More Questions? Consult the NIH

If you have further questions or need detailed information on vitamin K and its interactions, the National Institutes of Health (NIH) Office of Dietary Supplements is an excellent resource. Their comprehensive fact sheets provide science-based information on dietary supplements and their effects on health. Find their resources here: NIH Office of Dietary Supplements: Vitamin K Fact Sheet

Frequently Asked Questions

Warfarin works by inhibiting vitamin K, which is essential for blood clotting. If your vitamin K intake fluctuates, the level of warfarin needed to maintain the correct clotting speed also changes, risking either excessive bleeding or dangerous clot formation.

Yes, you can. The key is to eat them consistently, not to eliminate them. Maintaining a stable, predictable amount of vitamin K-rich foods each week helps your doctor accurately adjust your warfarin dosage.

You should use extreme caution and always consult your doctor before taking any new herbs or supplements. Many, such as ginseng, ginkgo biloba, and high-dose garlic, can interfere with blood clotting and pose a risk.

Not all multivitamins contain vitamin K, but many do. You should always read the label carefully and confirm the contents with your healthcare provider before starting a new multivitamin.

Prolonged use of antibiotics can destroy the 'good' gut bacteria that produce vitamin K, potentially lowering your body's levels of the vitamin. This is especially true for certain types of antibiotics like cephalosporins.

If you are on warfarin, it is generally recommended to avoid grapefruit and cranberry juice. While their primary interaction is with the metabolism of warfarin itself and not vitamin K, they still alter the drug's effectiveness and increase bleeding risk.

At high doses, vitamin E may interfere with vitamin K's blood-clotting activity by potentially blocking metabolic pathways. This can increase the risk of bleeding, especially for those on blood-thinning medications.