Nattokinase: The Science and the Hype Behind the "Natural Blood Thinner"

Nattokinase is having a moment. It shows up in longevity stacks, “natural blood thinner” listicles, and heart-health marketing, usually with a confident claim that it dissolves clots, clears arteries, and protects against strokes — all from a fermented-soy supplement.

The underlying science is real and genuinely interesting. But it’s also a textbook case of how a legitimate laboratory finding gets stretched into clinical claims it can’t support. Let me walk through both halves honestly: what nattokinase actually does, and where the hype outruns the evidence.

What Nattokinase Is

Natto is a traditional Japanese breakfast food — soybeans fermented with the bacterium Bacillus subtilis. It’s famously pungent and sticky, and it’s been eaten in Japan for centuries.

In 1987, the researcher Hiroyuki Sumi, publishing in Experientia, discovered that natto contains a potent enzyme capable of dissolving blood clots in the laboratory. He named it nattokinase. A few years later, in 1993, Fujita and colleagues purified and characterized the enzyme in Biochemical and Biophysical Research Communications, confirming its strong fibrinolytic activity — meaning it breaks down fibrin, the mesh-like protein that forms the structural scaffold of a blood clot.

That’s the foundational science, and it’s solid. Nattokinase is a real enzyme with a real, reproducible ability to chew through fibrin in a dish.

The Science: What’s Actually Supported

Here’s where precision matters, because “dissolves fibrin in a test tube” and “thins your blood when you swallow a capsule” are very different claims.

Fibrinolysis in the lab — strong. The in-vitro evidence that nattokinase degrades fibrin and fibrin-related proteins is well established. No serious dispute there.

Blood pressure in humans — modest, real. The single best piece of human evidence is a 2008 randomized, controlled trial by Kim and colleagues in Hypertension Research. Over eight weeks, participants taking nattokinase saw a modest reduction in blood pressure — roughly 5 mmHg systolic — compared to placebo. That’s a genuine, if small, randomized signal, and it’s the most credible human result the supplement has.

Cardiovascular markers — preliminary. A 2018 review by Chen and colleagues in Biomarker Insights gathered the broader literature, which includes small studies reporting effects on cholesterol, fibrinogen, and measures of carotid-artery thickening. The review is appropriately titled “a promising alternative” — promising being the operative word. These are small, often short, sometimes industry-linked studies, and the authors themselves call for larger, rigorous trials before any clinical conclusion.

So the fair summary of the science: a well-documented enzyme with strong test-tube activity, one modest blood-pressure trial, and a scattering of preliminary human signals that haven’t been confirmed in large, high-quality studies.

The Hype: Where the Claims Outrun the Evidence

Now the other half. Here’s what gets claimed that the evidence does not support:

• “Dissolves the clots that cause heart attacks and strokes.” There is no human outcome trial showing nattokinase prevents or treats heart attacks, strokes, deep-vein thrombosisthrombotic Relating to blood clots forming inside vessels — the trigger for many heart attacks and strokes., or pulmonary embolismpulmonary embolism A blood clot that has traveled to the lungs and blocked an artery there. A medical emergency.. The clot-dissolving claim is extrapolated from lab data, not demonstrated in patients.
• “Clears arterial plaque.” Atherosclerotic plaque is not a blood clot — it’s a complex deposit of cholesterol, inflammatory cells, and fibrous tissue built into the artery wall. An enzyme that degrades fibrin in a dish is not established to reverse plaque in a human artery. (For what actually drives plaque, see our pieces on ApoB and Lp(a).)
• “A natural alternative to blood thinners.” This one is both unproven and dangerous, because it invites people to swap a precisely-dosed, monitored prescription for an unregulated enzyme of variable potency. No serious clinician would make that trade.

There’s also a basic pharmacology question the hype ignores: how much intact, active enzyme actually survives stomach acid and digestion to reach the bloodstream? Nattokinase is a protein, and proteins are generally broken down in the gut. The degree to which oral nattokinase delivers meaningful systemic fibrinolytic activity in humans remains genuinely uncertain.

Clinical Callout: A useful tell for supplement hype: watch for claims that jump from a mechanism (“breaks down fibrin”) straight to an outcome (“prevents strokes”) with no human outcome trial in between. Nattokinase is a clean example. The mechanism is real; the outcome claim is a marketing leap. That gap — mechanism without proven outcomes — is exactly where most supplement overreach lives.

Safety: The Bleeding Risk Is Real

This is the part that most “natural blood thinner” content glosses over, and it’s the part I care most about as a pharmacist.

If nattokinase has any real anticoagulantanticoagulant A blood-thinning medicine that slows clotting — warfarin is one. Adding supplements that also affect clotting can change how it works. effect — and its entire selling point is that it does — then that effect carries the same risk any blood thinner carries: bleeding. Animal toxicity studies, like a 2019 evaluation by Wu and colleagues in Regulatory Toxicology and Pharmacology, suggest nattokinase itself is not acutely toxic or genotoxic at the doses tested. But “not toxic” is not the same as “safe to combine with everything.” The concrete cautions:

• Do not combine with anticoagulants or antiplatelets — warfarin, apixaban, rivaroxaban, clopidogrel, or even daily aspirin — without physician oversight. The additive bleeding risk is the real danger here, and there are case reports of serious bleeding associated with nattokinase.
• Stop well before surgery or dental procedures. Like fish oil and several other supplements, nattokinase should be discontinued in advance of anything that involves bleeding risk. Tell your surgeon and anesthesiologist you’ve been taking it.
• Avoid if you have a bleeding disorder, a history of hemorrhagic stroke, or active ulcers, and avoid in pregnancy, where it hasn’t been studied.
• Soy allergy is a contraindication, since the enzyme is derived from fermented soy.

Quality and Dosing — If You Still Want to Try It

If, after all that, you and your physician decide nattokinase is reasonable for you, two practical points:

• Potency is measured in fibrinolytic units (FU), not milligrams. Most studies and products cluster around 2,000 FU per day (roughly 100 mg). A label that lists only milligrams without an FU value is a quality red flag.
• Most supplements remove the vitamin K2. Whole natto is rich in vitamin K2, which itself affects clotting — so reputable nattokinase supplements are processed to remove it. This matters if you’re on warfarin (where K2 interferes with dosing), but it also means you’re not getting natto’s K2 benefits from the supplement.

Buy only from brands that do third-party testing and state an FU potency. The supplement market’s quality variance is real, and an enzyme product is only as good as its manufacturing.

The honest framing to carry away: nattokinase is a real enzyme with real laboratory activity and one modest blood-pressure trial — wrapped in marketing claims about clots, plaque, and stroke prevention that the human evidence simply hasn’t earned. Interesting? Yes. Proven heart protection? No. And unlike most supplements, this one can actually make you bleed — so it deserves a real conversation with your clinician, not an impulse buy.