Nattokinase is generally well tolerated in studies, and serious side effects are rare. The main safety concern is bleeding: nattokinase is an enzyme that breaks down fibrin, a protein involved in blood clotting, so it can add to the effect of blood thinners and raise the risk of bruising or bleeding. This guide covers the side effects to know about, what the research says about the liver, which medicines to check, and who should speak to a GP or pharmacist before taking a nattokinase supplement.
The quick answer
- Usually well tolerated: a 2016 toxicology review rated nattokinase of low toxicological concern, and a 3-year trial found no effect on any blood test it measured.
- Bleeding is the main risk: easy bruising, nosebleeds or bleeding gums, especially if you also take a blood thinner or aspirin.
- Do not combine it with blood thinners such as warfarin, apixaban, rivaroxaban, clopidogrel or aspirin unless your doctor agrees.
- Liver: liver damage is not a recognised side effect, but liver disease itself can affect clotting, so check with your specialist first.
- Stop at least 2 weeks before surgery or dental treatment, and tell your surgeon or dentist.
Is nattokinase safe?
For most healthy adults, nattokinase appears to be well tolerated. Nattokinase is made by the bacterium Bacillus subtilis var. natto, which ferments soybeans into the Japanese food natto. In a 2016 toxicology assessment, it was not mutagenic, 90-day studies in rats found no evidence of toxicity, and healthy volunteers tolerated it well over 4 weeks. The authors concluded that eating nattokinase is of low toxicological concern (Lampe and English, 2016).
The longest trial to date gave 265 healthy adults with a median age of 65 either 2,000 FU of nattokinase or a placebo for a median of 3 years. It found no significant effect on blood pressure or on any of the laboratory measures, which included blood clotting and fibrinolysis factors (Hodis et al., 2021).
That does not mean nattokinase suits everyone. Because it acts on the clotting system, the people who need to be careful are those who already bleed more easily or take medicines that affect clotting.
Common nattokinase side effects
Side effects are uncommon. These are the ones to look out for:
| Side effect | Why it happens | What to do |
|---|---|---|
| Easy bruising, nosebleeds, bleeding gums | Nattokinase acts on fibrin and clotting factors. | Stop taking it and speak to your GP or pharmacist, especially if you take any other medicine. |
| Stomach discomfort | Occasionally reported when taken on an empty stomach. | Take it with a glass of water, or with a light snack if needed. |
| Allergic reaction | Nattokinase comes from fermented soybeans, and our capsules contain soya. | Do not take it if you are allergic to soya. Get urgent help if you have swelling of the face or trouble breathing. |
Nattokinase and bleeding risk
In a small placebo-controlled study, a single 2,000 FU dose of nattokinase raised markers of fibrin breakdown and lengthened a clotting time test over the following hours in 12 healthy young men, although every result stayed within the normal range (Kurosawa et al., 2015). On its own, in a healthy person, that change is small. Added to a medicine that already thins the blood, it may not be.
The clearest warning comes from a 2008 case report. A man who took aspirin to prevent another stroke had a bleed in the brain after taking nattokinase daily for 7 days. Scans showed he also had tiny old bleeds in the brain (cerebral microbleeds). The doctors suggested nattokinase may raise the risk of brain bleeds in people who are already prone to bleeding and take another antithrombotic medicine (Chang et al., 2008).
Nattokinase side effects and the liver
Liver damage is not a recognised side effect of nattokinase. The 2016 toxicology assessment found no evidence of toxicity in its 90-day animal studies, and no liver problems were linked to nattokinase in the human studies in this guide.
There is one liver point that does matter. The liver makes most of the proteins the blood needs to clot, so people with liver disease, such as cirrhosis, can already bleed more easily. Adding an enzyme that acts on clotting is not a good idea without advice. If you have any liver condition, speak to your GP or liver specialist before taking nattokinase.
Medicines and supplements to check
This is the most important safety section. Always tell your GP, pharmacist or hospital team that you take nattokinase.
| If you take | Why it matters | What to do |
|---|---|---|
| Anticoagulants: warfarin, apixaban, rivaroxaban, edoxaban, dabigatran, heparin | The effects on clotting may add up and raise the risk of bleeding. | Do not take nattokinase unless your doctor or anticoagulation clinic agrees. |
| Antiplatelets: aspirin, clopidogrel, ticagrelor, prasugrel | The 2008 brain bleed case happened in a person taking aspirin. | Do not combine them without your doctor's advice. |
| Anti-inflammatory painkillers (NSAIDs) such as ibuprofen or naproxen | NSAIDs can cause stomach bleeding on their own. | Ask your pharmacist before taking them together regularly. |
| Blood pressure medicines | Some small trials reported lower blood pressure with nattokinase; the 3-year trial found no effect. | Mention it to your pharmacist and keep up any home blood pressure checks. |
| Other supplements that may affect clotting: serrapeptase, lumbrokinase, high-dose fish oil, ginkgo, garlic | Combining several products that act on clotting has not been studied. | Avoid stacking them, and check with a pharmacist. See nattokinase vs serrapeptase and lumbrokinase vs nattokinase. |
Vitamin K2, natto and warfarin
Natto, the food nattokinase comes from, is also naturally rich in vitamin K2 (menaquinone-7, or MK-7). Vitamin K helps the blood to clot, and warfarin works by blocking it, so vitamin K2 can make warfarin less effective. In one study, as little as 10 micrograms of MK-7 a day changed how healthy volunteers responded to a warfarin-type anticoagulant (Theuwissen et al., 2013).
That is why eating natto is a particular problem for anyone on warfarin. Our nattokinase capsules are vitamin K2-free, but that does not make them safe with warfarin: nattokinase itself still acts on clotting. Read more in our guide to nattokinase foods and natto.
How much nattokinase is safe?
There is no official UK or EU upper limit for nattokinase. Its strength is measured in fibrinolytic units (FU), which describe its activity rather than its weight. The 3-year trial used 2,000 FU a day.
Each of our capsules provides 6,000 FU, and the label serving is 1 to 2 capsules a day. If you are new to nattokinase, starting with one capsule is sensible. Do not exceed the stated daily dose, and do not take two nattokinase products at once. Our guides to nattokinase dosage and the best time to take nattokinase cover how to take it.
Who should not take nattokinase
Do not take nattokinase, or speak to your GP or pharmacist first, if you:
- take an anticoagulant or antiplatelet medicine, including daily aspirin
- have a bleeding disorder such as haemophilia, or bruise or bleed easily
- have had a stroke, a bleed in the brain or a stomach ulcer
- have liver disease or kidney disease
- take blood pressure medicines
- are allergic to soya
- are pregnant or breastfeeding, as there is not enough safety research
- have surgery or dental treatment planned: stop at least 2 weeks before and tell your surgeon or dentist
- want to give it to someone under 18
The bottom line on nattokinase side effects
Nattokinase is generally well tolerated, and safety studies have not found signs of toxicity, including in the liver. The real caution is bleeding: it acts on the clotting system, so it is not for anyone who takes blood thinners or aspirin, has a bleeding disorder or has surgery coming up. If you take any regular medicine, speak to your GP or pharmacist before you start, and stop and get advice if you notice unusual bruising or bleeding.
Vanessa holds a Level 3 Diploma in Nutrition and co-founded NutritionGo, where she leads editorial content. She researches and writes our guides, covering ingredient composition, traditional use and nutrition information, and fact-checks them against UK advertising rules and the GB Nutrition and Health Claims register. Every factual statement is verified against sources such as the NHS, the GB Nutrition and Health Claims register or peer-reviewed research on PubMed before publication. Vanessa is not a registered dietitian or healthcare professional, so this guide is for general information and is not a substitute for personal medical advice.
References
View all 6 references and sources
- Lampe BJ, English JC. Toxicological assessment of nattokinase derived from Bacillus subtilis var. natto. Food Chem Toxicol, 2016;88:87-99. Not mutagenic, no toxicity in 90-day rat studies, well tolerated by healthy volunteers over 4 weeks.
- Hodis HN, Mack WJ, Meiselman HJ, et al. Nattokinase atherothrombotic prevention study: a randomized controlled trial. Clin Hemorheol Microcirc, 2021;78(4):339-353. 265 adults, 2,000 FU a day for a median of 3 years; no effect on blood pressure or any laboratory measure.
- Kurosawa Y, Nirengi S, Homma T, et al. A single-dose of oral nattokinase potentiates thrombolysis and anti-coagulation profiles. Sci Rep, 2015;5:11601. Changes in clotting markers after one 2,000 FU dose, all within the normal range.
- Chang YY, Liu JS, Lai SL, et al. Cerebellar hemorrhage provoked by combined use of nattokinase and aspirin in a patient with cerebral microbleeds. Intern Med, 2008;47(5):467-9.
- Theuwissen E, Teunissen KJ, Spronk HM, et al. Effect of low-dose supplements of menaquinone-7 (vitamin K2) on the stability of oral anticoagulant treatment: dose-response relationship in healthy volunteers. J Thromb Haemost, 2013;11:1085-92.
- Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Nattokinase has no authorised health claims.