Nattokinase Side Effects

Nattokinase earns strong safety marks in clinical trials — but the same fibrinolytic activity that makes it useful for cardiovascular health creates real risks for specific groups of people.

The research is clear on two things: for most healthy adults at a standard 2,000 FU dose, nattokinase is well-tolerated with minimal side effects. But for anyone on blood thinners, scheduled for surgery, or with a clotting disorder, the risks are clinically significant and must be taken seriously.

This guide breaks down what the evidence actually says — including the adverse event data from the largest nattokinase clinical trial to date, the documented case reports, and a precise list of who should not take this supplement.

Table of Contents

1. The Clinical Safety Record: What Trials Actually Show
2. Common Side Effects at Standard Doses
3. Bleeding Risk: Understanding the Real Concern
4. Drug Interactions: What to Avoid and Why
5. Who Should Not Take Nattokinase
6. BioAbsorb Nattokinase: Formulated With Safety in Mind
7. Frequently Asked Questions
8. Conclusion

The Clinical Safety Record: What Trials Actually Show

The most reliable way to assess a supplement's safety is to look at controlled clinical data — not anecdotes, and not theoretical risks extrapolated from mechanism. For nattokinase, that data is reassuring at standard doses in low-risk populations.

The Nattokinase Atherothrombotic Prevention Study (NAPS), conducted by the University of Southern California, remains the largest and longest randomized controlled trial of nattokinase to date. Across 265 participants followed over three years, the trial found no significant difference in adverse events between the nattokinase and placebo groups. Dizziness was recorded in 1.52% of nattokinase participants versus 0% in placebo — a small, non-significant difference that remained within the noise of a large trial.

A 2024 systematic review and meta-analysis published in Nutrients pooled data from 6 RCTs involving 546 participants and rated the overall methodological quality of included studies as high. Its conclusion on safety: no notable adverse events were reported in any of the included studies. A 2021 Italian observational study of 153 patients with vascular disease using nattokinase as adjunctive therapy similarly found no serious bleeding events or clinically significant laboratory changes.

A formal toxicology assessment of nattokinase, published in Food and Chemical Toxicology, found the enzyme non-mutagenic and non-clastogenic in laboratory assays. In 90-day subchronic toxicity studies in rats, no adverse effects were observed at doses up to 1,000 mg/kg/day — far above any human supplementation range. A 4-week clinical study in healthy human volunteers found 10 mg/kg/day (roughly 700 mg for a 70 kg adult) well tolerated. The compound was rated of "low toxicological concern" for oral consumption.

This evidence provides reasonable confidence in nattokinase's short-term safety profile — but it comes with important caveats. Clinical trials typically exclude high-risk patients (those on anticoagulants, with bleeding disorders, or significant comorbidities), which means the safety data primarily reflects healthy or low-risk adults. Rare adverse events can also go undetected in trials of this size.

Common Side Effects at Standard Doses

For most adults taking nattokinase at the clinically studied dose of 100 mg / 2,000 FU daily, side effects are uncommon. The following represent the adverse effects that have been reported across trials and observational data, organized by how well-supported each is in the evidence.

Mild gastrointestinal symptoms. Minor nausea or abdominal discomfort have been occasionally reported in clinical settings but are uncommon and not consistently documented across trials. A 4-week clinical evaluation found no substantial effect on hematologic, renal, or hepatic function, and small increases in hematocrit, platelet count, and serum cholinesterase all remained within normal ranges.

Dizziness or mild blood pressure lowering. Nattokinase has demonstrated blood pressure-lowering effects in clinical trials — a benefit for most users, but one that may occasionally cause dizziness or lightheadedness in people whose blood pressure is already in the lower range. The NAPS trial recorded dizziness in 1.52% of nattokinase participants. This effect is dose-dependent and more likely if nattokinase is combined with antihypertensive medications.

Slight blood glucose increase. The 2024 Nutrients meta-analysis observed a small but statistically significant increase in blood glucose in the nattokinase group (mean difference = 0.40 mmol/L, p < 0.0001). The clinical significance of this finding is unclear — it was not associated with adverse outcomes in any of the included trials — but people managing blood sugar levels should be aware of it.

Allergic reactions (rare). Nattokinase is derived from fermented soybeans, and rare cases of late-onset anaphylaxis have been documented in individuals with existing natto or soy allergies. Poly-gamma-glutamic acid, a fermentation byproduct, has been identified as the likely trigger in skin testing. Anyone with a confirmed soy or natto allergy should not take nattokinase supplements.

Bleeding Risk: Understanding the Real Concern

Bleeding is the central safety concern with nattokinase, and it deserves a direct, unambiguous treatment — neither minimized nor overstated.

Nattokinase works by breaking down fibrin, the protein that forms blood clots. This fibrinolytic activity is precisely what makes it useful for cardiovascular health. But it also means that in the wrong context — especially when combined with other blood-thinning agents or in a person with an underlying clotting vulnerability — nattokinase can tip the balance toward excessive bleeding.

In healthy adults at standard doses, the clinical trial record does not substantiate significant bleeding risk. The Drugs.com clinical reference, drawing on multiple trials, states that the potential for increased bleeding risk with nattokinase "has not been clearly substantiated" in otherwise healthy individuals. Hematologic parameters in short-term trials have remained within normal limits.

However, case reports tell a different story in vulnerable populations. The most serious documented case is a 2021 report in Cureus by Ramachandran et al., describing an elderly woman who developed severe hemoperitoneum — internal abdominal bleeding — and subsequently died from complications attributed to nattokinase's anticoagulative properties. A separate case described acute cerebellar hemorrhage in a 52-year-old woman with a history of ischemic stroke who was taking daily aspirin and had added nattokinase at 400 mg/day — four times the standard dose. A third case documented valve thrombosis requiring reoperation in a patient who substituted nattokinase for warfarin after mechanical aortic valve replacement.

These cases are not population-level statistics — they are individual reports, and causality is difficult to establish with certainty in any single case. But they identify a consistent pattern: nattokinase poses elevated bleeding risk in people with pre-existing clotting vulnerabilities, those on concurrent blood-thinning medications, and those using doses significantly above 2,000 FU.

Warning signs that warrant immediate medical attention include unusual or prolonged bruising, blood in urine or stool, unexplained nosebleeds, and headache with neurological symptoms.

Drug Interactions: What to Avoid and Why

Drug interactions represent the most practically important safety consideration for people considering nattokinase. Unlike most of the side effect data — which applies primarily to edge cases — interaction risks apply to a large proportion of the cardiovascular patient population that nattokinase is most commonly marketed to.

Anticoagulants (warfarin, heparin, DOACs). Combining nattokinase with anticoagulants is the interaction of greatest concern. Both agents reduce clot formation through different mechanisms, and the combination can produce additive effects that significantly elevate bleeding risk. A clinical interaction database entry for nattokinase and warfarin rates this interaction as documented in both animal/lab research and human case reports. The case of the patient who substituted nattokinase for warfarin after valve replacement — with near-fatal thrombotic consequences — also illustrates the danger of replacing prescribed anticoagulants with nattokinase rather than using it as a complement. Nattokinase is not a pharmaceutical-grade anticoagulant and cannot reliably substitute for warfarin in high-risk cardiac patients. To learn more about how nattokinase functions as a natural blood thinner, see our dedicated article on the topic.

Antiplatelet drugs (aspirin, clopidogrel, ticagrelor). Nattokinase has demonstrated antiplatelet activity in laboratory and animal studies. The documented case of cerebellar hemorrhage occurred in a patient combining nattokinase with aspirin — the only concurrent blood-thinning agent. Low-dose aspirin (81 mg/day) for cardiovascular prevention may be acceptable in combination for some patients, but this assessment must be made by a physician who knows the individual's full cardiovascular history.

Antihypertensive medications. Nattokinase independently lowers blood pressure in clinical trials. When combined with antihypertensive drugs, there is a theoretical risk of additive blood pressure lowering that could cause dizziness or hypotension. This interaction is rated moderate, meaning caution is warranted rather than strict avoidance, but your prescribing physician should be aware.

NSAIDs (ibuprofen, naproxen). Non-steroidal anti-inflammatory drugs have their own mild antiplatelet effects. Regular NSAID use alongside nattokinase could modestly increase bleeding risk, though this combination is considered lower-risk than anticoagulant combinations. Occasional use is unlikely to be clinically significant; chronic use warrants physician review.

The practical rule: if you are on any medication that affects blood clotting or blood pressure, disclose nattokinase use to your prescribing physician before starting. This is especially important given that dosage and timing can influence the degree of fibrinolytic activity.

Who Should Not Take Nattokinase

Based on the clinical literature, the following groups should either avoid nattokinase entirely or use it only under direct physician supervision.

People on anticoagulant or antiplatelet medications. This is the most important contraindication. Warfarin, heparin, direct oral anticoagulants (DOACs such as rivaroxaban, apixaban, dabigatran), and antiplatelet drugs (clopidogrel, ticagrelor) all increase bleeding risk when combined with nattokinase. The Memorial Sloan Kettering Cancer Center integrative medicine database lists this as nattokinase's primary drug interaction concern.

People with bleeding disorders. Conditions such as hemophilia, von Willebrand disease, thrombocytopenia, or any coagulopathy create heightened baseline bleeding risk. Adding nattokinase's fibrinolytic activity in these populations is contraindicated based on mechanism, even without direct trial evidence.

People with a history of ischemic stroke or cerebral microbleeds. The documented case of cerebellar hemorrhage involved a patient with prior ischemic stroke. Nattokinase is contraindicated in individuals with ischemic stroke history, as noted in the Drugs.com clinical pharmacology reference.

People with peptic ulcers or gastrointestinal bleeding risk. Active peptic ulcer disease is a standard contraindication for fibrinolytic agents. Nattokinase is no exception.

People scheduled for surgery or dental procedures. Nattokinase should be discontinued before any surgical or invasive procedure. RxList advises stopping at least 2 weeks before scheduled surgery; other guidelines cite 7–14 days depending on the procedure's bleeding risk. There is no specific antidote to reverse nattokinase's fibrinolytic effects in an emergency, which is an important consideration. Always disclose nattokinase use during your pre-operative evaluation.

Pregnant and breastfeeding women. No human safety data exists for nattokinase use in pregnancy or lactation. Nattokinase's fibrinolytic properties raise theoretical concerns, and avoidance is recommended unless a physician has specifically assessed the individual risk and benefit.

Children under 18. No pediatric safety data exists. Nattokinase is not recommended for anyone under 18 without direct pediatric medical supervision.

People with soy or natto allergies. Nattokinase is derived from fermented soybeans. Anyone with a confirmed soy or natto allergy should not take nattokinase supplements due to the risk of allergic reaction, including the rare possibility of anaphylaxis.

BioAbsorb Nattokinase: Formulated With Safety in Mind

Understanding a supplement's side effect profile matters most when you can trust what's actually in the capsule. Dose inconsistency — nattokinase measured in fibrinolytic units (FU) rather than milligrams alone — is one of the key factors that makes safety comparisons across products difficult. A supplement labelled "nattokinase" with no FU activity listed may deliver far less enzyme activity than claimed, or far more.

BioAbsorb Nattokinase is formulated to match the dose used in clinical research: 100 mg per capsule, standardized to 2,000 FU of fibrinolytic activity. Every batch is third-party tested to verify that the activity level meets this specification — not just the milligram weight. This matters because FU activity is what determines the enzyme's physiological effect, and activity can vary significantly between raw material batches if not tested.

The formula uses DRcaps delayed-release veggie capsules — a phthalate-free alternative to standard enteric coatings that protects the enzyme from stomach acid degradation without the plasticizers found in conventional coated capsules. The enzyme reaches the small intestine intact, where absorption occurs.

One formulation decision deserves particular attention for safety-conscious users: BioAbsorb Nattokinase is intentionally free of Vitamin K2. Many nattokinase supplements include K2 as a companion nutrient. However, K2 can interfere with INR values in patients on warfarin — a concern that compounds the drug interaction risks discussed in this article. By excluding K2, this formula removes that confounding variable, making it easier for a physician to monitor any changes if supplementation is used alongside medical supervision.

Additional quality markers: manufactured in a Canadian GMP-certified facility, non-GMO, non-irradiated, and free of gluten, dairy, eggs, nuts, fish, shellfish, and animal products. See the product page for current pricing and available package sizes.

Frequently Asked Questions

Is nattokinase safe to take every day?

For healthy adults without contraindications, daily nattokinase at 2,000 FU appears well-tolerated based on clinical trial data, including the 3-year NAPS trial. Short-term safety data up to 7 months from multiple trials has not raised significant concerns in low-risk populations. Long-term safety data beyond 3 years is limited, so periodic reassessment with your healthcare provider is prudent for extended use.

Can nattokinase cause internal bleeding?

In healthy adults at standard doses, clinical trials have not documented significant bleeding risk. However, case reports have documented serious internal bleeding — including one fatal case of hemoperitoneum — in vulnerable individuals, particularly elderly patients or those with concurrent use of blood-thinning medications. The risk is real and clinically significant in high-risk groups, which is why contraindications must be taken seriously.

Can I take nattokinase with aspirin?

This combination requires physician guidance. Both nattokinase and aspirin have antiplatelet effects, and there is a documented case report of intracranial hemorrhage in a patient combining them. Low-dose aspirin (81 mg/day) prescribed for cardiovascular prevention may be acceptable in combination for some patients — but that decision should be made by a physician who knows your individual cardiovascular history, not self-directed.

How long before surgery should I stop taking nattokinase?

Most clinical guidelines recommend stopping nattokinase at least 2 weeks before any scheduled surgery or dental procedure. Some sources cite 7 days as sufficient for minor procedures. Because there is no specific antidote to reverse nattokinase's fibrinolytic effects if bleeding occurs during surgery, erring on the side of longer discontinuation is advisable. Always disclose supplement use during pre-operative screening.

Does nattokinase affect the liver or kidneys?

Short-term clinical data has not shown significant hepatic or renal effects at standard doses. A 4-week trial found no substantial changes in renal or hepatic function markers. Dedicated safety studies in patients with significant liver or kidney impairment have not been conducted, so caution is warranted in these populations and physician oversight is appropriate.

Does nattokinase interact with blood pressure medications?

Nattokinase independently lowers blood pressure, so combining it with antihypertensive drugs creates a moderate interaction risk — primarily additive blood pressure lowering that could cause dizziness or hypotension. This is not a strict contraindication but warrants disclosure to your prescribing physician, particularly if you are already on the lower end of your blood pressure target range.

Conclusion

Nattokinase has a genuinely good safety record in clinical trials — no notable adverse events across 546 participants in a recent meta-analysis, and a clean 3-year safety profile in the NAPS trial. For healthy adults at a standard 2,000 FU dose, the risks are low. But the enzyme's fibrinolytic mechanism creates real, documented risks for specific groups: those on anticoagulants or antiplatelets, those with bleeding disorders, and those preparing for surgery. If you fall outside those categories and want a rigorously tested, third-party verified nattokinase supplement, BioAbsorb Nattokinase is formulated at the clinically studied 100 mg / 2,000 FU dose, with DRcaps technology and no Vitamin K2.

Research References

1. Toxicological Assessment of Nattokinase Derived from Bacillus subtilis var. natto. Food and Chemical Toxicology, Vol. 88 (2016), pp. 87–99. Lampe, B.J. & English, J.C. GLP-compliant toxicology assessment finding nattokinase non-mutagenic, non-clastogenic, and of "low toxicological concern" at oral doses; NOAEL of 1,000 mg/kg/day established in 90-day rat study; 4-week human volunteer study at 10 mg/kg/day well tolerated.
2. Data Recorded in Real Life Support the Safety of Nattokinase in Patients with Vascular Diseases. Nutrients, Vol. 13, No. 6 (2021), p. 2031. Gallelli, L., Di Mizio, G., Palleria, C., et al. Observational study of 153 vascular disease patients using nattokinase 100 mg/day as adjunctive therapy; no serious bleeding events or clinically significant laboratory changes attributed to nattokinase during follow-up.
3. Nattokinase Atherothrombotic Prevention Study (NAPS): A Randomized Controlled Trial. Clinical Hemorheology and Microcirculation, Vol. 78, No. 4 (2021), pp. 339–353. Hodis, H.N., Mack, W.J., Meiselman, H.J., et al. Largest nattokinase RCT to date (n=265, 3 years); no significant difference in adverse events between nattokinase and placebo groups; adverse event data registered at ClinicalTrials.gov NCT02080520.
4. Nattokinase-Associated Hemoperitoneum in an Elderly Woman. Cureus, Vol. 13, No. 12 (2021), p. e20074. Ramachandran, L., Aqeel, A., Jafri, A., et al. Case report documenting severe internal abdominal bleeding and subsequent death in an elderly woman attributed to nattokinase's anticoagulative properties; highlights bleeding risk in vulnerable populations.
5. Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients, Vol. 16 (2024). Li, X., Long, J., Gao, Q., et al. PRISMA-compliant meta-analysis of 6 RCTs, 546 participants; no notable adverse events attributable to nattokinase; minor blood glucose increase (MD = 0.40 mmol/L) and blood pressure reductions observed.
6. Safety of Fermented Soybean Extract NSK-SD® as a Novel Food — EFSA Scientific Opinion. EFSA Journal, Vol. 14, No. 7 (2016). European Food Safety Authority regulatory safety opinion; confirmed non-mutagenicity, non-clastogenicity, and 90-day NOAEL of 1,000 mg/kg/day; identified fibrinolytic activity as the primary regulatory safety concern warranting monitoring.
7. Nattokinase — Clinical Pharmacology and Safety. Drugs.com Natural Products Database (2026). Aggregated clinical reference drawing on Hitosugi 2014, Health Canada 2012, Homma 2006, Hsia 2009, and Chang 2008; documents 4-week trial findings, hematologic parameters within normal limits, and the cerebellar hemorrhage case report.
8. Nattokinase — Integrative Medicine Database. Memorial Sloan Kettering Cancer Center (2024). Institutional integrative medicine reference; documents drug interaction risks with anticoagulants, antiplatelets, and fibrinolytic drugs; lists serious adverse event case reports including hemoperitoneum fatality and valve thrombosis.
9. Nattokinase — Supplement Monograph. RxList / WebMD Health (2021). Clinical supplement reference; establishes 2-week pre-surgical discontinuation guideline; rates antihypertensive drug combination as moderate caution; confirms pregnancy/lactation avoidance due to insufficient safety data.
10. Nattokinase–Warfarin Drug Interaction. HelloPharmacist Clinical Interaction Database (2024). Pharmacist-curated drug-supplement interaction entry; rates nattokinase-anticoagulant interaction as documented in animal/lab research and limited human case reports; cites intracranial hemorrhage case from nattokinase-aspirin combination.
11. Is Nattokinase a Blood Thinner? Effects and Risks. ScienceInsights (May 2026). Documents Stanford Medicine preoperative guidelines listing nattokinase among supplements requiring discontinuation 7 days before surgery; cites MSKCC guidance on clotting disorder contraindications.

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.