Nattokinase: Benefits, Side Effects and Bleeding Risks

Nattokinase is an enzyme made by the bacterium Bacillus subtilis when it ferments soybeans into natto, a traditional Japanese food. In lab tests it breaks down fibrin, the protein that holds blood clots together, which is why it is marketed for circulation and heart health. The human evidence is thin: small studies show changes in clotting markers, but a three-year randomized trial found no effect on artery thickening, blood pressure or lab measures. The most important fact is safety. Nattokinase can raise bleeding risk, especially with aspirin or blood thinners, and it is not a substitute for any prescribed anticoagulant. Here is what the research shows.

Peptide Labs sells dietary supplements, but we do not sell nattokinase, and this article does not recommend any product. It is general information, not medical advice.

What is nattokinase?

Japanese researchers described nattokinase in 1987 after finding strong clot-dissolving activity in natto. They named the enzyme, showed that it digested fibrin in laboratory tests, and found it easy to extract. Natto itself has been eaten in Japan for centuries and is known for its sticky texture and strong smell.

Supplements contain the isolated enzyme, usually in capsules, with strength often given in fibrinolytic activity units rather than weight. That makes products hard to compare, and the activity measured in a test tube does not tell you how much reaches the bloodstream intact. Natto is also one of the richest food sources of vitamin K, which matters for anyone taking warfarin; some nattokinase supplements say the vitamin K has been removed, so check the label rather than assuming.

Nattokinase benefits: what the research shows

Most claims about nattokinase benefits come from lab and animal work. Human research is limited, and the largest trial was negative.

• Clotting markers: a 2015 double-blind crossover study gave 12 healthy young men a single serving of nattokinase or placebo. Markers of clot breakdown such as D-dimer rose and a clotting time test lengthened over the next hours, but every value stayed within the normal range.
• Artery health: the Nattokinase Atherothrombotic Prevention Study at the University of Southern California randomized 265 adults with a median age of about 65 and no known heart disease. After a median of three years, artery wall thickening and stiffness did not differ from placebo, and nattokinase had no significant effect on blood pressure or any laboratory measure.
• Blood pressure and lipids: a few small trials have reported lower blood pressure, but a 2023 meta-analysis of six trials with 546 participants found mixed results across risk factors.
• Other claims: Memorial Sloan Kettering notes that claims for Alzheimer’s disease rely on animal studies and that clinical data for cancer claims are lacking.

Nattokinase side effects and bleeding risk

Nattokinase side effects are mostly about bleeding, because the enzyme affects the same clotting system that blood thinners target. Memorial Sloan Kettering Cancer Center advises that people with clotting disorders, and people taking anticoagulant, antiplatelet or clot-dissolving drugs, should avoid nattokinase, and that it may increase the effect of daily aspirin. Case reports it lists include:

• A bleed in the brain after a week of nattokinase in a patient already taking aspirin who had tiny old bleeds on brain scans (published in 2008).
• Fatal internal bleeding in an elderly woman who took over-the-counter nattokinase for an irregular heartbeat and was not taking other blood thinners.
• A blood clot on a mechanical heart valve that required repeat surgery after a patient replaced prescribed warfarin with nattokinase for a long period.
• Severe allergic reactions in people allergic to natto or fermented soy.

MSKCC also notes a theoretical risk that nattokinase could dislodge an existing clot, and says people with a history of deep vein thrombosis should avoid it. The valve case shows why nattokinase must never be used in place of warfarin, a newer anticoagulant or an antiplatelet drug. If you take any of these, do not stop them or add nattokinase without talking to the prescriber.

Who should talk to a clinician first

• Anyone taking warfarin, other anticoagulants, antiplatelet drugs, or daily aspirin.
• People with a bleeding or clotting disorder, a history of stroke, or a history of blood clots.
• Anyone scheduled for surgery or dental work; tell the team about every supplement you take.
• People combining it with other supplements that may affect bleeding, such as turmeric, ginger or fish oil.
• People who are pregnant or breastfeeding, and anyone with a soy allergy.

Seek urgent care for signs of serious bleeding, such as black or bloody stools, vomiting blood, a sudden severe headache, or weakness on one side of the body.

How to read a nattokinase label

Because nattokinase is sold as a dietary supplement, the FDA does not approve products for safety or effectiveness before they are sold. Manufacturers are responsible for making sure their products are safe and accurately labeled, and the FDA generally acts only after a product is on the market. That makes the label and independent testing your main tools, and it is why the clinical trials above, rather than marketing copy, are the fairest guide to what the enzyme does and does not do.

Look for the enzyme activity stated per serving, the source, an allergen statement for soy, and any statement about vitamin K. Combination products may add serrapeptase or other enzymes, which brings extra uncertainty; MSKCC reports a case of serious tissue damage after someone injected an oral enzyme supplement into a blood vessel, a reminder that capsules are only for swallowing. Independent testing helps confirm what is in the bottle; see our guide to third-party tested supplements. For a different supplement with a much milder safety profile, see our astaxanthin guide.

Frequently Asked Questions

Questions about a label? Contact us or read our Buyer’s Guide. Follow the label and ask a healthcare provider before starting any supplement, especially if you take medicines. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Last reviewed October 2026.

Related guides: Fermented Foods: List, Benefits and Live vs Pasteurized, Krill Oil vs Fish Oil: Absorption, Evidence and Safety Compared, Turmeric and Black Pepper: Why Piperine Is Added and What to Know.