- Bleeding and bruising risk is the single most consistently cited concern across medical sources, since nattokinase breaks down blood clots[1]
- Combining nattokinase with blood thinners (warfarin, aspirin, clopidogrel) may raise bleeding risk, although human interaction data are lacking [4][2]
- A case report: a 92-year-old woman not on any anticoagulant developed hemoperitoneum while taking nattokinase and later died, and the authors only suspect nattokinase contributed [5][3]
- A Health Canada licence says to stop nattokinase 7 days before scheduled surgery, and general advice for supplements is to stop 1 to 2 weeks before [6][7][8][2]
Nattokinase, the fermented-soybean enzyme marketed for cardiovascular and circulation support, has a consistent negative-review theme across medical literature: it’s not digestive upset or “didn’t work” complaints driving concern here, it’s bleeding risk.
“It did not work” is also a fair complaint on the evidence. A 3-year trial in 265 healthy adults found no slowing of carotid artery thickening and no effect on blood pressure, and a 2023 meta-analysis concluded that relatively low-dose nattokinase may have no significant lipid-lowering effect [12][9].
The Blood-Thinning Effect Is the Real Complaint
Nattokinase’s mechanism of action is exactly what makes it risky for some users: it promotes fibrinolysis, the breakdown of blood clots. Excessive fibrinolysis can increase bleeding complications, and even minor cuts or bruises may bleed more than usual, with internal bleeding possible in severe cases[1]. This isn’t a rare side effect buried in fine print, it’s the direct extension of the supplement’s advertised mechanism.
Dangerous Drug Interactions
The most repeated warning across sources: nattokinase should never be combined with prescription anticoagulants (warfarin, heparin, rivaroxaban, apixaban) or antiplatelet drugs (aspirin, clopidogrel, ticagrelor) without direct medical supervision, since the combination may add to bleeding risk, although human interaction data are lacking [4][9][2]. Even common over-the-counter drugs like ibuprofen carry the same caution[2].
Research note: In a 2023 trial of a nattokinase and red yeast rice supplement, no intervention-related adverse events were reported, and coagulation, liver and kidney function did not differ from placebo [13].
A Documented Serious Case Without Blood Thinners
One case report is worth knowing specifically because it didn’t involve any concurrent medication: a 92-year-old woman who was not taking blood thinners or aspirin, but was regularly ingesting nattokinase supplements in unclear amounts (“sometimes a handful”), experienced moderate hemoperitoneum after a fall. CT scanning revealed multiple bleeding spots in the liver[3]. A single case report cannot show cause: the authors only hypothesize that the bleeding was partly due to nattokinase, and the amount taken is unknown [5].
Surgery-Related Warnings
Multiple sources specifically warn that nattokinase might increase bleeding risk during or after surgery, and can make blood pressure difficult to control during a procedure. A Health Canada licence says to stop it 7 days before scheduled surgery, and general advice for herbal medicines and supplements is to stop 1 to 2 weeks before [6][7][8][2].
Milder, More Common Complaints
Beyond bleeding, users report digestive upset, tied to fermentation byproducts from the Bacillus subtilis strain used in production: a real concern for anyone with soy sensitivity, since the supplement is soy-derived[1]. Some also report dizziness or blood pressure dropping too low, although the trials reviewed here reported no adverse events attributed to nattokinase [9][10][1].
The Balancing Data Point
To be fair, research consistently shows nattokinase is generally well-tolerated at recommended doses, with serious side effects considered rare. In a 90-day rat study reviewed by EFSA, the no-adverse-effect level was 1,000 mg/kg body weight per day, the highest dose tested, with no significant effects on coagulation measures. That is animal data and does not rule out bleeding in people [11][1].
FAQ
Can nattokinase cause dangerous bleeding?
Yes, especially in combination with blood thinners or at high, unmonitored doses. A documented case involved serious internal bleeding even without concurrent medication[3].
Should I stop nattokinase before surgery?
Yes: a Health Canada licence says to stop 7 days before scheduled surgery, and general supplement advice is 1 to 2 weeks [6][7][8][2].
Is nattokinase safe with aspirin?
Not without medical supervision. Combining the two may raise bleeding risk, although human interaction data are lacking [4][2].
References
- EarthClinic. Nattokinase Side Effects: Risks, Interactions, and Dosage. www.earthclinic.com/supplements/nattokinase-s…
- WebMD. Nattokinase: Uses, Health Benefits and Side Effects. www.webmd.com/vitamins-and-supplements/nattok…
- Wellfit Insider. Nattokinase Dangers: Side Effects, Risks, & Who Should Avoid It. wellfitinsider.com/nutrition/nattokinase-dang…
- Chen H et al. Nattokinase: A Promising Alternative in Prevention and Treatment of Cardiovascular Diseases. Biomarker insights (2018). PMID 30013308
- Lintu Ramachandran et al. Nattokinase-Associated Hemoperitoneum in an Elderly Woman. Cureus (2021). PMID 35003944
- Health Canada Licensed Natural Health Product NPN 80089299: Nattokinase 100 mg. Health Canada (2026). source
- Elvir Lazo OL et al. Use of herbal medication in the perioperative period: Potential adverse drug interactions. Journal of clinical anesthesia (2024). PMID 38613937
- Hatfield J et al. Dietary supplements and bleeding. Proceedings (Baylor University. Medical Center) (2022). PMID 36304597
- Li X et al. Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Reviews in cardiovascular medicine (2023). PMID 39076715
- Jensen GS et al. Consumption of nattokinase is associated with reduced blood pressure and von Willebrand factor, a cardiovascular risk marker: results from a randomized, double-blind, placebo-controlled, multicenter North American clinical trial. Integrated blood pressure control (2016). PMID 27785095
- European Food Safety Authority (EFSA NDA Panel) Safety of fermented soybean extract NSK-SD as a novel food pursuant to Regulation (EC) No 258/97. European Food Safety Authority (EFSA NDA Panel) (2016). source
- Hodis HN et al. Nattokinase atherothrombotic prevention study: A randomized controlled trial. Clinical hemorheology and microcirculation (2021). PMID 33843667
- Liu X et al. The Effect of Nattokinase-Monascus Supplements on Dyslipidemia: A Four-Month Randomized, Double-Blind, Placebo-Controlled Clinical Trial. Nutrients (2023). PMID 37836525
These statements have not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease. Content is for informational purposes only and is not medical advice; consult a qualified healthcare provider before starting any supplement. As an Amazon Associate we earn from qualifying purchases.
