What are the disadvantages of nattokinase?

The largest and longest trial of nattokinase ever conducted followed 265 adults for three years and found no measurable effect on artery wall thickness.

That trial used 2,000 fibrinolytic units per day — the dose most supplements sell. It points to the first of several disadvantages of nattokinase that rarely make it into product marketing.

Here is the honest accounting: bleeding risk, drug interactions, metabolic trade-offs, and who has good reason to avoid this enzyme.

Table of Contents

• 1. Bleeding Risk: The Best-Documented Disadvantage
• 2. Drug Interactions Cut in Both Directions
• 3. The Efficacy Ceiling: What the Longest Trial Found
• 4. The Metabolic Trade-Offs Most Guides Skip
• 5. Who Should Not Take Nattokinase
• 6. Why Two "2,000 FU" Labels Are Not Equal
• 7. What Transparent Nattokinase Labelling Looks Like
• Frequently Asked Questions
• Conclusion
• Research References

1. Bleeding Risk: The Best-Documented Disadvantage

Nattokinase works by breaking down fibrin, the protein scaffold that holds a clot together. Mechanism reviews describe direct fibrinogen cleavage alongside a proposed effect on angiotensin-converting enzyme that remains unresolved in humans. The same action that makes the enzyme interesting is the source of its main disadvantage.

The clearest published harm signal is a 2008 case report. A 52-year-old woman taking daily aspirin for secondary stroke prevention developed acute cerebellar hemorrhage after 7 consecutive days of nattokinase at 400 mg. Brain MRI showed multiple cerebral microbleeds. The authors were careful: her stroke history means causality cannot be established from a single case.

Trial data looks reassuring by comparison, and that gap matters. Across the 6 randomized trials pooled in 2023, no notable adverse events were reported. A real-world study of 153 patients aged 22 to 92 taking 100 mg daily recorded no adverse drug reactions. But trials screen out the highest-risk patients, and 546 pooled participants is a small base from which to detect rare events.

• Trial doses cluster around 2,000 FU daily; the hemorrhage case involved 400 mg
• The 2023 meta-analysis pooled 546 participants — too few for rare-event detection
• The 153-patient safety study enrolled only people naïve to anticoagulants and antiplatelets
• Case reports of bleeding events number in the single digits, not zero

If you want the wider safety picture, our companion piece on nattokinase side effects and when to stop covers the same ground from the reader's decision point.

2. Drug Interactions Cut in Both Directions

Most articles frame the warfarin interaction as one-way: nattokinase adds blood-thinning on top of a blood thinner. The reality is messier. Memorial Sloan Kettering notes that natto is rich in vitamin K, and that Bacillus subtilis keeps synthesizing vitamin K in the intestine after it is eaten — which works against warfarin rather than with it.

Whether that applies to a capsule depends on manufacturing. Professional monographs note that vitamin K2 can lower INR and may persist in supplements if it is not removed during production. Two products at the same 2,000 FU can therefore behave differently in someone on a vitamin K antagonist.

The substitution risk is separate and more serious. A patient with a mechanical aortic valve replaced warfarin with nattokinase and developed valve thrombosis after nearly a year, requiring repeat valve replacement. Nattokinase does not deliver the anticoagulation a mechanical valve requires. Stacking with prescription antithrombotics carries its own risks, which we work through in our guide to nattokinase as a natural blood thinner.

3. The Efficacy Ceiling: What the Longest Trial Found

A disadvantage does not have to be a side effect. It can simply be that the supplement does less than expected, and here the strongest evidence is not favourable. The Nattokinase Atherothrombotic Prevention Study randomized 265 adults of median age 65.3 to 2,000 FU or placebo. After a median of 3 years, the annualized rate of change in carotid intima-media thickness and arterial stiffness did not significantly differ between groups.

Blood pressure is where the evidence holds up better, though the effect is modest. The 2023 meta-analysis found reductions of 3.45 mmHg systolic and 2.32 mmHg diastolic in the three pooled trials that measured blood pressure, covering 223 participants. Real, reproducible, and well short of what a prescription antihypertensive delivers.

What remains missing is the endpoint that matters most. No randomized trial of nattokinase has demonstrated a reduction in heart attack, stroke, or cardiovascular death — the studies measure surrogate markers over 8 weeks to 3 years. Our article on nattokinase for heart health works through that null result in detail.

4. The Metabolic Trade-Offs Most Guides Skip

This is the disadvantage almost nobody writes about, and it comes from the same meta-analysis usually cited to support nattokinase. Alongside the blood pressure benefit, pooled data showed blood glucose rose by a mean difference of 0.40 relative to placebo (95% CI 0.20 to 0.60, p<0.0001) in the two trials that measured it, with no heterogeneity between them.

The lipid findings run the same direction at lower total dosages. Across the four pooled trials that measured blood lipids in 399 participants, lower-dose nattokinase was associated with a mean difference of +5.27 in total cholesterol, +6.49 in LDL, and −2.76 in HDL compared with control. At higher total dosage, total cholesterol was still higher by 3.18, though HDL and LDL differences lost significance. Triglycerides showed no relationship at all (p=0.71).

These are small shifts in pooled surrogate markers, not evidence of harm, and the review's authors still concluded nattokinase has a role as adjunctive therapy for hypertension. But if you are taking it partly to improve cholesterol, the 399-participant lipid evidence points the other way — and that is worth knowing before you spend 6 months on it.

5. Who Should Not Take Nattokinase

Professional guidance is specific. Nattokinase is not recommended for people with ischemic stroke, peptic ulcer, or coagulation disorders, for anyone on concomitant anticoagulant therapy, or in the period before or after surgery. That last one catches people out — an enzyme with fibrinolytic activity has no reversal agent comparable to vitamin K for warfarin.

Allergy is an underappreciated exclusion. Fermented soybean products cause an unusual late-onset anaphylaxis driven by poly-gamma-glutamic acid, a by-product of the fermentation process. In one documented case involving a patient already sensitised to natto, symptoms appeared roughly 3 hours after ingesting a food containing that same compound. Because poly-gamma-glutamic acid can carry over into supplements, professional guidance is that anyone with a history of hypersensitivity to natto should avoid nattokinase products. The delayed reaction makes the cause easy to miss.

• Anyone taking warfarin, heparin, a direct oral anticoagulant, aspirin or clopidogrel
• People with a mechanical heart valve, where 1 documented substitution led to valve thrombosis
• Anyone scheduled for surgery, both before and after the procedure
• People with a history of natto or fermented soybean allergy
• Pregnant or nursing women, for whom controlled data is absent

Duration is a separate question from eligibility. The longest controlled trial ran 3 years, and beyond that there is no controlled evidence in either direction — we set out what is and is not established in our complete guide to nattokinase.

6. Why Two "2,000 FU" Labels Are Not Equal

Nattokinase potency is measured in fibrinolytic units, not milligrams, because the number that matters is enzyme activity rather than powder weight. The 3-year NAPS trial specified 2,000 FU, while the 153-patient safety study specified 100 mg daily. A label quoting only milligrams tells you very little.

Vitamin K2 content is the second variable, and it is invisible on most labels. Since K2 may remain in a supplement if it is not removed during production, two products with identical FU claims can differ in how they interact with warfarin. For anyone on a vitamin K antagonist, that single manufacturing decision changes the risk profile.

Dose also interacts with the metabolic findings above, where the 2023 pooled analysis separated lower from higher total dosage and found different lipid signals in each. If you are working out what to take, our nattokinase dosage guide sets out the studied ranges.

7. What Transparent Nattokinase Labelling Looks Like

The disadvantages above are mostly properties of the enzyme itself, and no brand engineers those away. What a manufacturer does control is how much uncertainty it hands you.

The BioAbsorb Nattokinase Enzyme is built around removing three of them. Each capsule delivers 100 mg standardised to 2,000 FU — activity stated, not just weight — matching the range used in the clinical literature. Every batch is third-party tested for FU activity at 2,000 or above, plus heavy metals, gluten and microbial contaminants, so the label figure is verified rather than assumed.

The formulation is deliberately free of vitamin K2. Residual K2 is the mechanism by which a nattokinase supplement can work against warfarin, so its absence removes one variable from that conversation. Since this is an article about disadvantages, the other side deserves stating plainly: professional monographs record that Health Canada has raised the concern that removing vitamin K2 could theoretically increase bleeding risk, on the reasoning that natto's native K2 may partly offset it, and that current supplement safety data do not settle the question either way. The evidence does not establish which effect dominates. What it does establish is that anyone taking an anticoagulant needs a physician in the loop regardless of which formulation they choose.

Delivery uses DRcaps delayed-release capsules, which protect the enzyme from stomach acid without the phthalates and plasticizers found in conventional enteric coatings. The product is manufactured in a Canadian GMP-certified facility, non-GMO, non-irradiated, and free of gluten, nuts, eggs, dairy, fish, shellfish and all animal products. At 180 capsules for $49.87, a 6-month supply works out near $0.28 per day.

None of that makes nattokinase right for everyone. It makes the product a known quantity, which is what you need in order to make the decision properly.

Frequently Asked Questions

Does nattokinase thin your blood too much?

In controlled trials it has not. Across 6 randomized trials and 546 participants, no notable adverse events were reported. The concern applies mainly to people whose clotting is already altered by medication or existing vascular disease.

Can you take nattokinase with aspirin?

Not without medical supervision. The one published hemorrhage case involved nattokinase added to daily aspirin in a patient with cerebral microbleeds. Combining two agents that act on different parts of the clotting system compounds the effect of either alone.

Should you stop nattokinase before surgery?

Yes. Professional guidance advises against use before or after surgery, though it does not specify how long beforehand to stop. Tell your surgical team you have been taking it and let them set the timing, since it will not appear on your medication list.

Does nattokinase raise blood sugar?

Pooled trial data suggests a small increase. The 2023 meta-analysis found a mean difference of 0.40 versus placebo in the two trials that measured glucose, with no heterogeneity between them. The shift is modest, but it is worth monitoring if you already track glucose.

Can nattokinase cause an allergic reaction?

It can in people sensitised to fermented soybeans. Natto allergy characteristically produces late-onset anaphylaxis several hours after ingestion, driven by poly-gamma-glutamic acid from the fermentation process. The delay makes the cause easy to miss.

Is nattokinase safe for long-term use?

The longest controlled evidence runs 3 years. The NAPS trial followed 265 participants over that period without a safety signal emerging, though it also found no benefit on its primary endpoint. Beyond 3 years, there is no controlled data either way.

Conclusion

The disadvantages of nattokinase are real but specific: a bleeding risk that concentrates in people already on antithrombotic therapy, interactions that run in both directions, small unfavourable shifts in glucose and lipids, and a 3-year trial at 2,000 FU that found no effect on artery wall thickness. If you have weighed those and want a product where potency and formulation are verified rather than assumed, review the BioAbsorb Nattokinase Enzyme — and speak with your physician first if you take any medication affecting clotting.

Research References

• Nattokinase atherothrombotic prevention study: A randomized controlled trial. Clinical Hemorheology and Microcirculation, Vol. 78 (2021). Randomized 265 adults to 2,000 FU nattokinase or placebo; after a median 3 years, carotid intima-media thickness and arterial stiffness did not differ significantly from placebo.
• Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Reviews in Cardiovascular Medicine, Vol. 24 (2023). Pooled 6 RCTs and 546 participants; found blood pressure reductions in 3 trials, a small increase in blood glucose in 2 trials, and unfavourable lipid shifts at lower total dosage in 4 trials.
• Cerebellar Hemorrhage Provoked by Combined Use of Nattokinase and Aspirin in a Patient with Cerebral Microbleeds. Internal Medicine, Vol. 47 (2008). Reported acute cerebellar hemorrhage in a 52-year-old woman after 7 days of nattokinase at 400 mg daily while taking aspirin.
• Consequence of patient substitution of nattokinase for warfarin after aortic valve replacement with a mechanical prosthesis. Proceedings (Baylor University Medical Center), Vol. 28 (2015). Documented mechanical valve thrombosis after approximately one year of nattokinase used in place of warfarin.
• Data Recorded in Real Life Support the Safety of Nattokinase in Patients with Vascular Diseases. Nutrients, Vol. 13 (2021). Followed 153 patients aged 22 to 92 on 100 mg daily, all naïve to anticoagulant and antiplatelet therapy, and recorded symptom improvement without adverse drug reactions or drug interactions.
• Late-onset anaphylaxis due to poly(gamma-glutamic acid) in the soup of commercial cold Chinese noodles in a patient with allergy to fermented soybeans (natto). Allergology International, Vol. 60 (2011). Identified poly-gamma-glutamic acid as the allergen behind delayed anaphylaxis occurring hours after ingestion.
• Nattokinase: A Promising Alternative in Prevention and Treatment of Cardiovascular Diseases. Biomarker Insights, Vol. 13 (2018). Mechanism review covering fibrinogen cleavage and the unresolved status of proposed ACE inhibition in humans.
• Nattokinase. Memorial Sloan Kettering Cancer Center, Integrative Medicine Herbs Database. Notes vitamin K content in natto and continued intestinal vitamin K synthesis as a mechanism reducing warfarin's effect.
• Nattokinase — Professional Natural Products Monograph. Drugs.com. Sets out contraindications including ischemic stroke, peptic ulcer, coagulation disorders, concomitant anticoagulant therapy, and the perioperative period; also records Health Canada's concern that removing vitamin K2 from supplements could theoretically increase bleeding risk.

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.