Search this question and the very first page of results hands you 2 flatly opposite answers. Rank 2 describes a clot and a repeat valve replacement, rank 3 reports no major adverse events.
Neither page reconciles them, so this one does. Both are accurate, and 331 records explain why.
Quick Answer: What Are Nattokinase's Side Effects?
In the 10 clinical trials, side effects were mild and the 2024 meta-analysis reported no major adverse events at any dose. Outside the trials, 7 case reports describe serious harm, including a fatal bleed into the abdomen, a cerebellar haemorrhage in someone also taking aspirin, and anaphylaxis in a person allergic to natto. Trials enrolled relatively healthy volunteers; the harms happened to people who were older, anticoagulated or already ill.
Key Takeaways
- The 2024 meta-analysis reported 0 major adverse events across the trials.
- The same database holds 7 case reports, and 1 patient died.
- The 3 mild effects reported are nausea, bloating and loose stools.
- 1 case of anaphylaxis is published, in someone allergic to natto.
- The 2 groups differ: trial volunteers were healthier than the case patients.
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The Two Halves of the Record
This is the part almost every page gets wrong by picking a side, so it is worth laying out before anything else.
- Half one: 10 clinical trials, 2 meta-analyses, mild effects only.
- Half two: 7 case reports, including 1 death.
- Who was in half one: Mostly healthy volunteers, on about 2000 FU.
- Who was in half two: Older people, often already on anticoagulants.
What the enzyme is and where it comes from is in what nattokinase actually is.
What the Trials Reported
The trial record really is reassuring, and it deserves to be stated without hedging before the other half arrives.
- Meta-analysis finding: No major adverse events at any dose tested.
- Largest trial: 265 adults, median age 65.3, long term.
- Commonly reported: Mild nausea, bloating or loose stools.
- Liver and kidney: No significant changes reported in the trials.
A 2024 systematic review and meta-analysis of randomized trials is the source of the no-major-adverse-events line that the search results quote.[1]Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials — Reviews in Cardiovascular Medicine (2024) View source It is a summary of trials, and trials are not where the harms turned up.
What the Case Reports Describe
Seven case reports exist. These 4 are the ones that change a decision.
| Case report | What happened | Year |
|---|---|---|
| Hemoperitoneum, elderly woman | Bleeding into the abdomen, and the patient died | 2022 |
| Warfarin substitution | Swapped warfarin for nattokinase after a mechanical valve | 2015 |
| Cerebellar haemorrhage | On aspirin, after 7 days of nattokinase, with microbleeds | 2008 |
| Anaphylaxis | In a patient with an existing fermented soybean allergy | 2022 |
The fatal case involved an elderly woman who developed bleeding into the abdomen and died.[2]Nattokinase-Associated Hemoperitoneum in an Elderly Woman — Cureus (2022) View source A separate patient on aspirin for stroke prevention had a cerebellar haemorrhage after 7 days of nattokinase, with cerebral microbleeds already present.[3]Cerebellar hemorrhage provoked by combined use of nattokinase and aspirin in a patient with cerebral microbleeds — Internal Medicine (2008) View source
The interaction cases are taken apart properly in the anticoagulant problem.
Liver, Kidneys and the Questions People Search
Both appear in related searches, so both deserve a direct answer rather than silence.
- Liver: No signal in the trials, and 0 case reports of liver injury.
- Kidney: Same picture, with 0 reported cases.
- What is reported instead: Bleeding, low blood pressure and allergy.
- Why that matters: The real risk is not the organ people worry about.
People search for liver damage because that is the familiar supplement worry. With this one the documented harms are almost entirely bleeding-related instead.
Allergy, and the One Absolute Exclusion Here
Nattokinase comes from fermented soybeans, and that origin carries a specific risk.
- If you react to natto: Do not take the enzyme, in any amount.
- If you react to soy: Treat it the same way until a clinician says otherwise.
- What is published: 1 case of anaphylaxis from nattokinase.
- Milder signs: Rash, itching or breathing difficulty.
The published case involved a patient with an existing fermented soybean allergy.[4]Anaphylaxis from nattokinase in a patient with fermented soybean (natto) allergy — Allergology International (2022) View source Where the enzyme sits relative to the food it comes from is in the vitamin K2 combination.
Why Both Halves Are True
The two records are not in conflict once you look at who was in each.
- Trials: Screened volunteers, excluded people on anticoagulants.
- Case reports: The people trials exclude, taking it unsupervised.
- Dose: Trials used about 2000 FU, retail products go to 10800 FU.
- Conclusion: Safety here is about who you are, not about the molecule.
For someone outside the excluded groups, our nattokinase capsules carry the amount that was actually studied.
Do Not Start Nattokinase, and Speak to a Clinician First, If
- You take warfarin, a DOAC, aspirin, clopidogrel or any other anticoagulant or antiplatelet
- You have a mechanical heart valve, a stent or any implanted cardiac device
- You have a bleeding disorder such as haemophilia, or a history of haemorrhagic stroke
- You have surgery or a dental procedure booked within the next 2 weeks
- You are pregnant, breastfeeding, or allergic to soy or natto
Frequently Asked Questions
What are the side effects of nattokinase? +
In the 10 trials, mild digestive effects and 0 major adverse events. Outside them, 7 case reports describe bleeding, allergy and 1 death.
Why don't doctors recommend nattokinase? +
Because it affects clotting and 4 of the 7 case reports involve bleeding or a clot. Prescribers cannot easily monitor it the way they monitor warfarin.
Does nattokinase damage the liver? +
There is no signal in the trials and 0 published cases of liver injury. The documented harms are bleeding-related instead.
Does nattokinase affect the kidneys? +
No kidney signal has been reported in the trials, and there are 0 case reports. It is not the organ this supplement puts at risk.
Has anyone died from nattokinase? +
1 published case report describes an elderly woman who developed hemoperitoneum and died. That is a single case, not a rate.
Can nattokinase cause low blood pressure? +
It can lower blood pressure, and combined with blood-pressure medication that may cause dizziness. That is 1 of the 3 main reported effects.
Is nattokinase safe for healthy people? +
The trial record in healthy adults is reassuring, with 0 major adverse events reported. The harms occurred in people the trials would have excluded.
Can I be allergic to nattokinase? +
Yes. 1 case of anaphylaxis is published, in a patient with an existing fermented soybean allergy.
Does nattokinase cause bruising? +
Easy bruising is 1 plausible early sign given the mechanism. Any new bruising is a reason to stop and ask, not to wait.
How long do side effects last? +
Mild digestive effects usually settle within the first 2 weeks. Anything involving bleeding is not something to wait out.
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