What are the negative side effects of nattokinase?

The most serious side effect ever documented for nattokinase was a fatal internal bleed in an elderly woman who was taking it on its own, with no other blood thinner.

That is one published case, and it sits alongside a 3-year trial in 265 adults [1] that found no significant safety signal at all. But three serious case reports define the shape of the risk, and they are worth understanding before you start.

Table of Contents

1. The Short Answer: What Is Actually Documented
2. Mild and Common: Digestive Upset, Headache, and Bruising
3. Bleeding: The Side Effect With Documented Harm
4. Drug Interactions That Turn a Mild Effect Serious
5. Who Should Not Take Nattokinase at All
6. Soy, Allergy, and Late-Onset Anaphylaxis
7. Product Quality: The Risk That Isn't in the Enzyme
8. Reducing the Avoidable Risks Through Formulation
9. Frequently Asked Questions
10. Conclusion
11. Research References

1. The Short Answer: What Is Actually Documented

Nattokinase has a reassuring short-term trial record and a narrow but real set of serious risks. The Nattokinase Atherothrombotic Prevention Study randomized 265 adults of median age 65.3 years to 2,000 fibrinolytic units daily or placebo for a median of 3 years [1], and found no significant effect on blood pressure or on any laboratory determination — including coagulation and fibrinolysis markers.

A 2023 systematic review pooled 6 randomized controlled trials with 546 participants [2] and rated the overall methodological quality of the included studies as high. Separately, a 4-month randomized trial in 113 patients with dyslipidemia [6] found no significant differences in coagulation, liver function, or renal function — though that trial tested a nattokinase-monascus combination that also contained 7 mg of lovastatin from red yeast rice, so its findings are not attributable to nattokinase alone.

The honest reading is that roughly 900 trial participants is not enough to detect rare adverse events. Every serious harm in the nattokinase literature comes from case reports rather than trials — and while most of those involve a drug interaction, the most severe one does not. For the broader risk-benefit picture, see our overview of whether there is a downside to taking nattokinase.

2. Mild and Common: Digestive Upset, Headache, and Bruising

The side effects most people actually notice are minor and self-limiting. Across published clinical studies, the mild effects recorded include diarrhea, constipation, stomach pain, headache, and in one instance an abnormal liver function test. A 12-month clinical study of 1,062 participants taking up to 10,800 FU daily [7] reported no noticeable adverse effects, including no significant changes in liver or kidney function.

Digestive symptoms are strongly tied to how the capsule is taken. Nattokinase is standardly dosed at 100 mg (2,000 FU) once daily on an empty stomach, and a minority of users report mild nausea or bloating under those conditions — usually resolved with a full glass of water or a shift in timing. Our nattokinase dosage guide covers the 30–60 minute pre-meal window in detail.

Easier bruising is the other commonly reported effect, and it is mechanistically plausible rather than mysterious: nattokinase reduces platelet aggregation in addition to its fibrinolytic action. Bruising that appears without obvious cause is worth reporting to your physician rather than ignoring, because it is the earliest visible signal of the risk covered in Section 3. Symptoms readers frequently attribute to the enzyme but which trials have not linked to it — such as heart palpitations and joint pain — are covered separately.

3. Bleeding: The Side Effect With Documented Harm

Nattokinase's entire therapeutic rationale is that it breaks down fibrin and supports the body's clot-dissolving system. That same action is the source of its only serious documented side effect. The most severe published case is a 2021 report of hemoperitoneum — bleeding into the abdominal cavity — in an elderly woman taking over-the-counter nattokinase for atrial fibrillation [4]. She received multiple transfusions, continued to decompensate, and died a week later. She was not taking any other anticoagulant.

That case matters because it is the exception to the pattern. The better-known 2008 report describes acute cerebellar hemorrhage in a 52-year-old woman taking nattokinase 400 mg daily for 7 consecutive days [3] while already using aspirin for secondary stroke prevention. Three details shape that one: the dose was four times the 100 mg standard serving, she had multiple cerebral microbleeds visible on MRI, and she was on an antithrombotic drug at the same time. As clinical monographs note, causality cannot be firmly established [9] given that history.

Taken together, the risk concentrates in people on antithrombotic medication — but does not appear to be confined to them. Memorial Sloan Kettering flags [8] that nattokinase may theoretically increase bleeding risk when combined with anticoagulant, antiplatelet, or fibrinolytic drugs, and separately notes a theoretical risk that fibrinolytic activity could dislodge an existing clot. Our detailed review of nattokinase as a natural blood thinner examines these mechanisms in full.

4. Drug Interactions That Turn a Mild Effect Serious

The interaction that carries the most weight is with anticoagulant and antiplatelet medication: warfarin, direct oral anticoagulants such as apixaban and rivaroxaban, heparin, aspirin, and clopidogrel. Effects are additive rather than competing, and routine INR monitoring does not measure nattokinase's fibrinolytic contribution at all — so a patient can sit inside their therapeutic INR range and still be carrying additional, unmeasured bleeding risk.

Substitution is the more dangerous error. A 2015 case report documents a patient who replaced warfarin with nattokinase 100 mg daily after mechanical aortic valve replacement [5]; after nearly a year, he developed thrombus on the valve and required repeat valve replacement surgery. The authors described it as the first documented case of its kind and strongly discouraged the practice.

There is a second, less obvious wrinkle. Natto the food contains high concentrations of vitamin K2, which reduces INR when taken with warfarin [9] — an effect that runs opposite to nattokinase's own. Most purified supplements remove K2 during processing, so that natural counterweight disappears. Anyone on any medication affecting coagulation should treat nattokinase as a prescribing decision, not a personal one.

5. Who Should Not Take Nattokinase at All

Health Canada's licensing framework is specific here. Licensed Canadian nattokinase products carry mandatory risk statements [10] instructing consumers not to use the product if pregnant or breastfeeding, if taking health products that affect blood coagulation, or if they have a bleeding disorder — and to consult a practitioner if they have a cardiovascular, kidney, or liver disorder, or if they are taking any medication.

The same labelling requires users to stop the product 7 days before any scheduled surgery, or immediately in the event of unscheduled surgery or an injury causing bruising or bleeding. Tell your surgeon and anesthesiologist you have been taking it, because it will not appear on a medication list unless you volunteer it.

Beyond the label, clinical sources add ischemic stroke, peptic ulcer, and mechanical heart valves to the avoid list, and advise against use before or after surgery. Memorial Sloan Kettering also advises that patients with a history of deep vein thrombosis should avoid nattokinase [8], on the theoretical concern that fibrinolytic activity could dislodge an existing clot. Pregnancy is a data gap rather than a demonstrated harm: there are no teratogenicity or obstetric outcome studies at all. If you are wondering specifically about hepatic risk, we address that in whether nattokinase is hard on your liver.

6. Soy, Allergy, and Late-Onset Anaphylaxis

Nattokinase is extracted from fermented soybeans, so every product in the category is soy-derived. Canadian licensed products carry an explicit allergen statement directing anyone with a soy allergy not to use them, and this applies regardless of how purified the enzyme is or how few milligrams of soy protein remain.

There is a rarer and stranger reaction worth knowing about. Natto can cause late-onset anaphylaxis, with symptoms appearing 4 to 15 hours after ingestion [9] — far outside the minutes-to-an-hour window most people associate with allergic reactions. Skin testing has implicated poly-gamma-glutamic acid, a by-product of the fermentation process, as the trigger.

Poly-gamma-glutamic acid content can be reduced during manufacturing, but not reliably eliminated across all products. Clinical guidance is that anyone with a history of hypersensitivity to natto should avoid nattokinase supplements entirely. [8] The delayed timing means a reaction may not be connected to the capsule taken half a day earlier, which is a reason to note new symptoms against your dosing schedule during the first 2 weeks.

7. Product Quality: The Risk That Isn't in the Enzyme

A meaningful share of nattokinase risk has nothing to do with nattokinase. The enzyme does not hold Generally Recognized as Safe status with the US Food and Drug Administration [9], and in 2012 Health Canada determined that safety concerns — including the potential for increased bleeding risk — had not been adequately addressed to allow marketing. Health Canada's specific concern was that because natto's own vitamin K2 content may partly offset bleeding risk, removing K2 during manufacturing could theoretically increase it. Available safety data still do not resolve that question. Regulatory posture on this ingredient is cautious on both sides of the border.

Potency is the practical problem. Nattokinase activity is measured in fibrinolytic units, not milligrams, and milligram weight alone tells you nothing about how active a given capsule is. Clinical doses in the published literature span 2,000 FU to 10,800 FU daily [7] — a greater than fivefold range — so a product that does not state FU, or does not verify it, is not comparable to any trial.

The second quality issue is what else is in the bottle. Blended cardiovascular formulas frequently combine nattokinase with serrapeptase, ginkgo, garlic, or high-dose fish oil, each of which carries its own antiplatelet activity. Stacking several mild blood-thinning agents in one capsule produces a cumulative effect that no single-ingredient trial has ever measured.

8. Reducing the Avoidable Risks Through Formulation

Some risks in this article are formulation problems rather than enzyme problems, and those are the ones a manufacturer can address. BioAbsorb Nattokinase Enzyme [12] delivers 100 mg standardized to 2,000 FU per capsule — the same daily dose used in the 265-participant NAPS trial [1] and in most of the blood pressure literature, rather than an unverified milligram figure.

It is a single-ingredient product rather than a blend, so there is no hidden stacking of additional antiplatelet botanicals. The formulation is also free of vitamin K2, which matters if you already supplement K2 separately or are discussing warfarin with a physician. We want to be precise about that last point rather than oversell it: as Section 7 notes, Health Canada's stated concern runs the other way — that removing K2 could theoretically increase bleeding risk. K2-free is a formulation choice with real relevance to some readers, not a safety guarantee for any of them.

Delivery uses DRcaps delayed-release veggie capsules, which protect the enzyme through gastric transit without the phthalates and plasticizers found in some enteric coatings. Every batch is third-party tested to confirm activity of at least 2,000 FU per capsule, along with screening for heavy metals, gluten, and microbial contaminants — which addresses the potency-verification gap described in Section 7. The product is manufactured in a Canadian GMP-certified facility [11], is non-GMO and 100% vegetarian, and is free of gluten, nuts, eggs, dairy, fish, and shellfish. Bottle sizes and current pricing are listed on the product page.

One caution we will not formulate away: this is a soy-derived enzyme, and the fatal case in Section 3 involved a woman on no other medication. If you have a soy allergy, a bleeding disorder, take any anticoagulant, or have surgery scheduled within 7 days, this product is not appropriate for you regardless of how it is made — and if you have a cardiac or clotting condition, this is a conversation for your physician before it is a purchase.

Frequently Asked Questions

How common are nattokinase side effects?

Uncommon at standard doses. A meta-analysis of 6 trials and 546 participants [2] rated the included trials as high quality, and the 3-year NAPS trial in 265 adults found no significant laboratory differences versus placebo. Mild digestive symptoms and easier bruising are the effects most often reported in practice. Serious events appear only in case reports, which trials of this size are not large enough to detect.

Has anyone died from taking nattokinase?

One published case. An elderly woman taking over-the-counter nattokinase for atrial fibrillation developed hemoperitoneum and died a week later [4], despite transfusion. She was not on any other anticoagulant, which is what makes the case significant — it is the only serious report that cannot be explained by a drug interaction. A single case report cannot establish how often this happens, but it does establish that it can.

Do I need to stop nattokinase before surgery?

Yes. Canadian licensed product labelling [10] instructs users to stop at least 7 days before any scheduled surgery, and immediately in the case of unscheduled surgery or an injury causing bruising or bleeding. Tell your surgeon and anesthesiologist that you have been taking it.

Can I take nattokinase with aspirin?

Not without physician approval. Memorial Sloan Kettering flags a theoretical increase in bleeding risk [8] when nattokinase is combined with antiplatelet drugs, and the documented cerebellar hemorrhage case involved exactly this combination. Daily low-dose aspirin still counts.

Can nattokinase replace my blood thinner?

No, and attempting it has caused documented harm. A patient who substituted nattokinase for warfarin after mechanical valve replacement developed valve thrombosis within about a year [5] and required repeat surgery. Nattokinase is not an equivalent to prescription anticoagulation.

How long can I safely take nattokinase?

The longest controlled evidence available is the NAPS trial, which dosed 2,000 FU daily for a median of 3 years [1] without a safety signal. Beyond that dose and duration, long-term data thin out quickly, and Canadian labelling directs anyone with a cardiovascular, kidney, or liver disorder — or taking any medication — to consult a practitioner. Ongoing use is a decision to review periodically with your own physician.

Conclusion

Nattokinase's negative side effects are mostly mild and infrequent, but its serious documented risk — bleeding — is real, most concentrated in people taking antithrombotic medication, and in one fatal case present without any interacting drug at all. Choosing a single-ingredient product with verified 2,000 FU potency removes the avoidable variables and leaves only the ones your physician should be weighing with you. Compare the specifications on BioAbsorb Nattokinase Enzyme.

Research References

1. Nattokinase atherothrombotic prevention study: A randomized controlled trial. Clinical Hemorheology and Microcirculation, Vol. 78, Issue 4 (2021), pp. 339–353. Double-blind trial randomizing 265 adults of median age 65.3 years to 2,000 FU nattokinase or placebo for a median of 3 years; found a null effect on subclinical atherosclerosis progression and no significant effect on blood pressure or any laboratory determination.
2. Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Reviews in Cardiovascular Medicine, Vol. 24, Issue 8 (2023). Pooled 6 randomized controlled trials with 546 participants; rated overall methodological quality of included studies as high and reported a significant reduction in systolic blood pressure.
3. Cerebellar Hemorrhage Provoked by Combined Use of Nattokinase and Aspirin in a Patient with Cerebral Microbleeds. Internal Medicine, Vol. 47, Issue 5 (2008), pp. 467–469. Case report of acute cerebellar hemorrhage following 7 consecutive days of nattokinase 400 mg daily in a 52-year-old woman on aspirin for secondary stroke prevention with multiple cerebral microbleeds on MRI.
4. Nattokinase-Associated Hemoperitoneum in an Elderly Woman. Cureus, Vol. 13, Issue 12 (2021), e20074. Case report of an elderly woman taking over-the-counter nattokinase for atrial fibrillation who developed hemoperitoneum, received multiple transfusions, continued to decompensate, and died one week later. She was not taking any other anticoagulant.
5. Consequence of patient substitution of nattokinase for warfarin after aortic valve replacement with a mechanical prosthesis. Proceedings (Baylor University Medical Center), Vol. 28, Issue 1 (2015), pp. 81–82. Documents mechanical valve thrombosis requiring repeat valve replacement after nearly a year of self-substituting nattokinase for warfarin.
6. The Effect of Nattokinase-Monascus Supplements on Dyslipidemia: A Four-Month Randomized, Double-Blind, Placebo-Controlled Clinical Trial. Nutrients, Vol. 15, Issue 19 (2023), 4239. Trial in 113 patients reporting no significant differences in coagulation, liver function, or renal function. Note: the intervention was a combination product containing 2,500 FU nattokinase plus 7 mg lovastatin derived from red yeast rice.
7. Effective management of atherosclerosis progress and hyperlipidemia with nattokinase: A clinical study with 1,062 participants. Frontiers in Cardiovascular Medicine, Vol. 9 (2022), 964977. Twelve-month study at doses up to 10,800 FU daily reporting no noticeable adverse effects, including no significant changes in liver or kidney function. A corrigendum was published in December 2022 (Front Cardiovasc Med 9:1076420).
8. Nattokinase. Memorial Sloan Kettering Cancer Center — About Herbs, Botanicals & Other Products. Clinical summary flagging theoretical increased bleeding risk with anticoagulant, antiplatelet, and fibrinolytic drugs, theoretical clot dislodgement, avoidance in patients with a history of deep vein thrombosis or natto hypersensitivity, and case reports including fatal internal bleeding.
9. Nattokinase — Professional Monograph. Drugs.com. Details late-onset anaphylaxis to natto occurring 4 to 15 hours after ingestion attributed to poly-gamma-glutamic acid, the absence of FDA GRAS status, contraindications including ischemic stroke and peptic ulcer, and Health Canada's 2012 determination regarding unaddressed bleeding risk concerns and vitamin K2 removal.
10. Licensed Natural Health Products Database — Nattokinase, NPN 80093953. Health Canada. Risk information for a licensed Canadian nattokinase product (100 mg / 2,000 FU delayed-release capsule): contraindicated in pregnancy and breastfeeding, bleeding disorders, and concurrent use of products affecting blood coagulation; soy allergen statement; discontinue 7 days before scheduled surgery.
11. BioAbsorb Nutraceuticals — Official Website. Canadian GMP-certified manufacturing, non-GMO formulation standards, and third-party batch testing protocols.
12. Nattokinase Enzyme — 100 mg / 2,000 FU / DRcaps / Veggie Capsules. BioAbsorb Nutraceuticals product page. Product specifications including standardized 2,000 FU activity, vitamin K2-free single-ingredient formulation, delayed-release DRcaps delivery, and allergen profile.

About the Author

David Kimbell is a health writer, digital entrepreneur and former aerospace engineer, based in Ottawa, Canada. He loves translating complex science into clear, actionable guidance for consumers seeking evidence-based solutions.

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Important Disclaimers

Medical Disclaimer: This article provides educational information only and is not intended as medical advice. Always consult with a qualified healthcare provider before starting any new supplement, especially if you have existing health conditions, take medications, or are pregnant or nursing.

FDA/Health Canada Statement: These statements have not been evaluated by the Food and Drug Administration or Health Canada. This product is not intended to diagnose, treat, cure, or prevent any disease.