What Supplements May Cause Blood Clots? A Pharmacological Review

October 6, 2025 •

4 min read

While both iron deficiency and excess have been linked to an increased risk of thromboembolic events [1.8.1], several other common supplements can also impact blood coagulation. This article explores the important question: what supplements may cause blood clots?

Quick Summary

Certain dietary supplements can increase the risk of blood clots by promoting coagulation or by interfering with anticoagulant medications. Key supplements of concern include Vitamin K, St. John's Wort, iron, and Coenzyme Q10.

Key Points

Vitamin K is a Pro-Coagulant: This essential nutrient helps produce clotting factors and can directly counteract the effects of blood thinners like warfarin [1.3.2, 1.3.4].
St. John's Wort Causes Interactions: It can reduce blood levels of warfarin, making the medication less effective and increasing the risk of clots [1.4.5].
CoQ10 May Interfere: Due to its similarity to vitamin K, CoQ10 might reduce the effectiveness of blood thinners, although evidence is conflicting [1.9.1, 1.9.2].
Iron's Dual Role: Both deficiency and excess of iron have been linked to a higher risk of thromboembolic events [1.8.1].
Consult a Professional: Always discuss supplements with a doctor or pharmacist, especially if you have clot risk factors or take anticoagulants [1.7.3].
Know the Emergency Signs: Recognizing symptoms of DVT (leg swelling, pain) and PE (sudden shortness of breath, chest pain) is critical for seeking timely medical help [1.10.1, 1.10.4].
Consistency is Key: For those on warfarin, maintaining a consistent daily intake of vitamin K from all sources is more important than complete avoidance [1.7.1].

Understanding Blood Clots and Supplement Risks

A blood clot, or thrombus, is a natural response to injury that prevents excessive bleeding [1.3.2]. However, when clots form in deep veins (deep vein thrombosis or DVT), they can be dangerous. A piece of a DVT can break off and travel to the lungs, causing a life-threatening pulmonary embolism (PE) [1.10.1]. While many factors contribute to clot risk, some dietary supplements can influence the body's clotting mechanisms. They typically do so in two main ways: by directly promoting the synthesis of clotting factors or by reducing the effectiveness of anticoagulant medications (blood thinners) like warfarin [1.2.2, 1.4.5]. Individuals with a history of blood clots, cardiovascular conditions, or those taking blood thinners must be especially cautious [1.2.3].

Key Supplements That May Increase Clot Risk

While many supplements are safe for the general population, some carry a specific risk related to blood coagulation. It is crucial to discuss any supplement use with a healthcare provider, particularly if you have underlying health conditions or are on medication [1.7.3].

Vitamin K

Vitamin K is famously known as the “clotting vitamin” [1.3.3]. It is essential for producing several proteins, called clotting factors, that are necessary for blood to clot properly [1.3.2]. For this reason, high-dose vitamin K supplements can directly counteract the effects of blood thinners like warfarin, making the medication less effective and increasing the risk of clot formation [1.3.1, 1.3.4]. People on warfarin are advised to maintain a consistent daily intake of vitamin K from both food and supplements to keep their blood clotting time stable [1.7.1].

St. John's Wort

This popular herbal supplement, often used for depression, poses a significant risk for those on anticoagulants [1.4.3]. St. John's wort can speed up the breakdown of many drugs in the body, including warfarin [1.4.2]. This interaction reduces the levels of the blood thinner in the bloodstream, making it less effective and leaving the individual vulnerable to developing blood clots [1.4.5]. The interaction is considered highly clinically significant, and combining the two is generally not recommended [1.4.1].

Coenzyme Q10 (CoQ10)

CoQ10 is structurally similar to vitamin K, and some reports suggest it may decrease the effectiveness of warfarin, thereby increasing the risk of blood clots [1.7.1, 1.9.1]. Although the data is conflicting, with some studies showing no interaction, the potential risk is serious enough that medical supervision is recommended when combining CoQ10 with blood thinners [1.7.1, 1.9.2, 1.9.4]. Health authorities like the Mayo Clinic and Merck Manuals list this potential interaction as a key safety concern [1.9.1, 1.9.5].

Iron

Iron's role in thrombosis is complex. Both iron deficiency and iron overload have been associated with an increased risk for blood clots [1.8.1]. High levels of iron from supplementation can promote oxidative stress and inflammation, which may contribute to clot formation [1.2.2]. Genetically determined higher iron status has been associated with an increased risk of venous thromboembolism [1.8.2]. Unless an iron deficiency is diagnosed, it is often best to get iron from dietary sources rather than high-dose supplements [1.2.2].

Other Herbal Supplements

Ginseng: Some studies and case reports suggest that ginseng can reduce the anticoagulant effects of warfarin, potentially increasing clot risk [1.7.4, 1.7.1].
Green Tea: In very large quantities (e.g., a gallon per day) or as concentrated extracts, green tea can interfere with warfarin due to its vitamin K content [1.7.1, 1.7.2].
Phytoestrogens (e.g., Dong Quai, Red Clover): Prescription estrogen is a known risk factor for blood clots. There is a theoretical risk that supplements with strong estrogen-like effects could pose a similar threat, especially in those with a pre-existing prothrombotic tendency [1.5.2, 1.6.4]. Dong quai, for example, has estrogenic effects that could potentially cause thrombosis [1.5.5].

Comparison of Supplements and Clotting Mechanisms


Supplement	Primary Mechanism of Risk	Primary Population at Risk	Citation
Vitamin K	Directly promotes synthesis of clotting factors.	Individuals taking warfarin.	[1.3.1]
St. John's Wort	Reduces the effectiveness of anticoagulant medication.	Individuals taking warfarin and other metabolized drugs.	[1.4.2, 1.4.5]
Coenzyme Q10	May decrease the effectiveness of warfarin.	Individuals taking warfarin.	[1.9.1, 1.9.5]
Iron (in excess)	Promotes oxidative stress and inflammation.	General population, particularly those with genetic predisposition.	[1.2.2, 1.8.2]
Ginseng	May reduce the anticoagulant effect of warfarin.	Individuals taking warfarin.	[1.7.1]

Recognizing a Blood Clot: When to Seek Medical Help

Knowing the symptoms of a blood clot is vital. A Deep Vein Thrombosis (DVT) often occurs in the leg and may present with:

Swelling, pain, or tenderness [1.10.1]
Skin that is warm to the touch [1.10.2]
Redness or discoloration [1.10.1]

A Pulmonary Embolism (PE) is a medical emergency. Symptoms include:

Unexplained shortness of breath [1.10.4]
Sharp chest pain, often worse with a deep breath [1.10.2]
Coughing, sometimes with blood [1.10.5]
Dizziness or fainting [1.10.2]

If you experience these symptoms, seek immediate medical attention.

Conclusion

While dietary supplements are widely used, they are not without risk. Supplements like vitamin K, St. John's wort, CoQ10, and high-dose iron can potentially increase the risk of blood clots, either directly or by interfering with life-saving anticoagulant medications. This highlights the critical importance of consulting with a healthcare professional before starting any new supplement, especially for individuals with a history of clots or those taking blood thinners. Full disclosure of all supplements to your medical team is a crucial step in ensuring your safety and preventing dangerous interactions.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before making any decisions about your health or treatment.

For more information on vitamin and mineral supplements, you can visit the NIH Office of Dietary Supplements.

Frequently Asked Questions

Generally, a standard multivitamin is unlikely to cause blood clots in a healthy person. However, if you take a blood thinner like warfarin, you should check if your multivitamin contains Vitamin K and discuss it with your doctor to ensure your intake remains consistent [1.7.1, 1.7.4].

Supplements that can decrease the effect of warfarin, increasing clot risk, include Vitamin K, St. John's Wort, Coenzyme Q10, ginseng, and large amounts of green tea [1.7.2, 1.7.4].

Not necessarily. 'Natural' does not mean 'safe.' Herbal supplements like St. John's Wort, ginseng, and dong quai can have powerful biological effects and cause dangerous interactions with medications, including increasing blood clot risk [1.4.3, 1.5.5, 1.7.1].

No. Dissolving existing, significant blood clots requires powerful prescription medications (thrombolytics) or medical procedures. No dietary supplement has been proven to be effective for this purpose.

Prescription estrogen is a known risk factor for blood clots. While data on phytoestrogens is less clear, there is a theoretical concern that they could increase clot risk, particularly in women with a history of thrombosis or other risk factors [1.5.2, 1.6.4].

St. John's Wort increases clot risk indirectly. It induces liver enzymes that break down medications like warfarin more quickly, lowering the drug's concentration in the blood and reducing its anticoagulant effect [1.4.2, 1.4.5].

Yes, many supplements should be discontinued before surgery. Supplements that affect clotting, such as ginger, ginkgo, and vitamin E (which can increase bleeding), are often stopped 1-2 weeks prior to a procedure to prevent complications [1.2.1].

A DVT (Deep Vein Thrombosis) is a blood clot that forms in a deep vein, usually in the leg [1.10.1]. A PE (Pulmonary Embolism) occurs when a piece of that clot breaks off, travels through the bloodstream, and gets lodged in the arteries of the lungs, which can be fatal [1.10.1, 1.10.3].