Nattokinase: The Fibrinolytic Enzyme for Microclots

Key Takeaways:

• Nattokinase has demonstrated fibrinolytic activity in both laboratory and human studies — it directly degrades fibrin and also enhances the body’s own fibrinolytic system by activating tissue plasminogen activator (tPA) and degrading plasminogen activator inhibitor-1 (PAI-1)
• The connection to post-COVID microclots is mechanistically rational but not yet validated by clinical trials — this is an important research direction, not established therapy
• Moderate evidence from human trials supports modest blood pressure reduction and improved blood viscosity parameters
• Nattokinase carries genuine bleeding risk and must not be combined with anticoagulants, antiplatelet agents, or taken perioperatively without physician oversight
• The combination with serrapeptase (“nattokinase-serrapeptase stack”) is popular in the post-COVID community but has minimal clinical evidence as a combination

At a Glance

Nattokinase and Microclots: What the Evidence Actually Shows

If you are reading this article, there is a reasonable chance you found it because of post-COVID. The connection between nattokinase and microclots has become one of the most discussed topics in the long COVID community, and I understand why. Patients who have been told their symptoms are psychosomatic, who have been dismissed by specialists, who are desperate for something that addresses the pathology they believe is driving their suffering — they hear about microclots and nattokinase and it sounds like an answer.

I want to give you an honest assessment. Not dismissive, not promotional. Here is what the evidence shows, where the gaps are, and what a reasonable approach looks like.

The Enzyme: What Nattokinase Is

Nattokinase is a serine protease enzyme produced by Bacillus subtilis natto during the fermentation of soybeans to produce natto — a traditional Japanese food that has been consumed for over a thousand years. The enzyme was first identified and characterized by Dr. Hiroyuki Sumi at the University of Chicago in 1987, when he placed natto on a fibrin plate and observed the clot dissolving.

Despite its name, nattokinase is not a kinase (an enzyme that phosphorylates proteins). It is a protease — an enzyme that cleaves proteins. Specifically, it cleaves fibrin, the structural protein that forms the meshwork of blood clots. It was named before its mechanism was fully understood, and the misnomer stuck.

Nattokinase has a molecular weight of approximately 27.7 kDa and demonstrates optimal activity at physiological pH and temperature. It is remarkably stable compared to many enzymes, retaining activity through a range of conditions including partial resistance to gastric acid — which is why oral supplementation can deliver some enzymatically active nattokinase to the bloodstream.

Mechanism of Action: How It Works

Nattokinase exerts fibrinolytic effects through multiple mechanisms. This is important to understand because it is more than a simple “clot dissolver.”

Direct Fibrinolysis

Nattokinase directly cleaves cross-linked fibrin — the mature, polymerized form of fibrin that constitutes the structural scaffold of blood clots. In laboratory studies, nattokinase demonstrates comparable fibrinolytic activity to plasmin, the body’s primary endogenous fibrinolytic enzyme. It cleaves fibrin at specific peptide bonds, generating fibrin degradation products (including D-dimer, which is clinically measurable).

Indirect Fibrinolytic Enhancement

Beyond direct fibrin degradation, nattokinase enhances the body’s own fibrinolytic system through two important mechanisms:

Activation of tissue plasminogen activator (tPA). tPA converts plasminogen to plasmin, the body’s primary clot-dissolving enzyme. Nattokinase has been shown to increase circulating tPA levels and activity.

Degradation of plasminogen activator inhibitor-1 (PAI-1). PAI-1 is the primary inhibitor of the fibrinolytic system. Elevated PAI-1 levels are associated with impaired clot resolution and have been documented in COVID-19 and long COVID. By degrading PAI-1, nattokinase removes a brake on the fibrinolytic system.

Anticoagulant Effects

Nattokinase has demonstrated anticoagulant activity independent of its fibrinolytic effects, including modest inhibition of Factor VII and Factor VIII. This adds to the overall antithrombotic profile but also contributes to bleeding risk.

The Microclot Connection: Post-COVID Context

What Are Microclots?

Microclots — more precisely described as amyloid fibrin(ogen) microclots or anomalous fibrin deposits — were identified in long COVID patients by Dr. Etheresia Pretorius and colleagues at Stellenbosch University beginning in 2021. This research, which our colleague Dr. Beate Jaeger has been closely following and contributing to in the context of impaired microcirculation, represents an important direction in understanding post-COVID pathology.

These microclots differ from normal blood clots in several ways:

• They are resistant to normal fibrinolysis (the body’s clot-dissolving process)
• They contain misfolded fibrinogen with amyloid-like properties
• They trap inflammatory molecules, including alpha-2-antiplasmin (which further inhibits fibrinolysis, creating a vicious cycle)
• They are associated with impaired microcirculation — blood flow through the smallest blood vessels
• They correlate with symptoms including fatigue, brain fog, exercise intolerance, and shortness of breath

The microclot hypothesis for long COVID proposes that these abnormal fibrin deposits impair microcirculation in multiple organ systems, reducing oxygen delivery and creating localized ischemia (oxygen deprivation) and endothelial dysfunction. This would explain the multi-system, fluctuating nature of long COVID symptoms.

Where Nattokinase Fits

The mechanistic logic connecting nattokinase to microclots is straightforward:

1. Microclots are composed of abnormal fibrin
2. Nattokinase degrades fibrin
3. Therefore, nattokinase might help dissolve microclots

Additionally:

4. PAI-1 is elevated in long COVID, impairing endogenous fibrinolysis
5. Nattokinase degrades PAI-1
6. Therefore, nattokinase might restore the body’s own ability to clear microclots

This is a rational hypothesis. Let me be clear about what it is not: it is not established therapy. No randomized controlled trial has demonstrated that nattokinase resolves microclots in long COVID patients. The connection is mechanistic, not clinical.

The Current Evidence Gap

As of early 2026, the evidence specifically for nattokinase in long COVID consists of:

• Case reports and case series describing symptom improvement in long COVID patients taking nattokinase, often in combination with other agents (aspirin, serrapeptase, bromelain)
• The FLCCC protocol and similar practitioner-developed protocols that include nattokinase as part of a multi-agent anticoagulant/fibrinolytic regimen, based on clinical observation rather than controlled trials
• Laboratory studies demonstrating that nattokinase can degrade spike protein-induced fibrin anomalies in vitro
• Ongoing clinical trials evaluating nattokinase for post-COVID symptoms (several registered on ClinicalTrials.gov as of 2026)

What we do not yet have: a completed, peer-reviewed RCT demonstrating that nattokinase improves microclot burden or clinical outcomes in long COVID patients. This is the evidence we need, and it is coming — but it is not here yet.

What I tell my patients: the rationale is sound, the preliminary signals are encouraging, and the risk profile at standard doses is acceptable for most patients. But I frame this as a reasonable clinical decision based on mechanism and observation, not as established therapy. The nuance matters.

Dr. Jaeger’s Research Context

At our hospital, Dr. Beate Jaeger has been investigating impaired microcirculation and endothelial dysfunction in post-COVID patients. Her work on microclots, combined with therapeutic apheresis approaches, represents an important research direction. The combination of macroscopic interventions (apheresis to physically remove abnormal proteins and inflammatory mediators from the blood) with oral fibrinolytic support (nattokinase, serrapeptase) and endothelial support is the kind of multi-modal approach that complex post-COVID presentations often require.

I want to frame this carefully: Dr. Jaeger’s work, and the broader microclot research field, represents an important research direction that is reshaping our understanding of post-COVID pathology. It is not yet settled science, and we approach it with scientific rigor and appropriate caution while remaining open to what the data shows us.

The Evidence: Beyond Post-COVID

What We Know (Human Data)

Setting aside the post-COVID application (where controlled trial data is limited), nattokinase has a broader evidence base for cardiovascular parameters.

Fibrinolytic activity in humans. Kurosawa et al. (2015) conducted an RCT in 45 healthy volunteers comparing nattokinase (2,000 FU daily) to placebo. The nattokinase group showed significantly increased fibrinolytic activity (measured by euglobulin clot lysis time and D-dimer), decreased fibrinogen levels, and decreased Factor VII and Factor VIII coagulant activities.

Blood pressure. Kim et al. (2008) conducted an 8-week RCT in 86 participants with initial systolic blood pressure of 130-159 mmHg. Nattokinase (2,000 FU daily) significantly reduced systolic blood pressure by an average of 5.55 mmHg and diastolic blood pressure by 2.84 mmHg compared to placebo. A 2016 meta-analysis by Ren et al. confirmed modest but statistically significant blood pressure reduction across studies.

Blood viscosity. Pais et al. (2006) demonstrated that nattokinase supplementation reduced blood viscosity and red blood cell aggregation in human subjects. Given that increased blood viscosity impairs microcirculation, this is relevant to the microclot hypothesis independent of direct fibrinolysis.

Atherosclerosis. A 26-week trial by Hsia et al. (2009) in 82 participants showed that nattokinase (6,000 FU daily) was associated with a reduction in carotid plaque size compared to statin therapy alone. However, this was an open-label study and needs confirmation.

What We See in the Lab (Preclinical)

Thrombolytic activity. In animal models of thrombosis, nattokinase has demonstrated significant thrombolytic (clot-dissolving) activity. Fujita et al. (1995) showed that oral nattokinase dissolved chemically induced thrombi in rat carotid arteries.

Antiatherosclerotic effects. In ApoE-knockout mice (a model for atherosclerosis), nattokinase supplementation reduced atherosclerotic plaque progression.

Neuroprotective effects. Emerging preclinical data suggests nattokinase may have direct neuroprotective properties beyond its vascular effects, potentially through reduction of neuroinflammation and improved cerebral microcirculation.

What I See in Practice

In our clinical experience treating post-COVID patients, nattokinase is part of a broader protocol that typically includes therapeutic apheresis, hyperthermia, and targeted supplementation. Patients who receive this comprehensive approach frequently report improvement in fatigue, brain fog, and exercise tolerance.

Attributing specific outcomes to nattokinase alone within a multi-modal protocol is not possible. What I can say is that patients who add nattokinase (typically 4,000 FU daily) to their regimen and maintain it consistently report subjective improvement in circulation-related symptoms — cold extremities, post-exertional symptom worsening, and cognitive clarity.

I am transparent with patients that this is clinical observation, not controlled evidence. But consistent clinical observation in a hospital that has treated thousands of post-COVID patients is worth something, even if it is not the same as an RCT.

Practical Application

Dosing

Standard dose: 2,000 FU (fibrinolytic units) daily, typically taken on an empty stomach. This is the dose used in most clinical trials for cardiovascular parameters.

Enhanced dose (often used in post-COVID protocols): 4,000 FU daily, split into two doses (2,000 FU morning, 2,000 FU evening). Some practitioners use up to 6,000 FU daily.

Timing: Empty stomach, at least 30 minutes before food or 2 hours after eating. Food, particularly protein-rich food, may reduce enzymatic activity. Some protocols specify morning and bedtime dosing.

Duration: In clinical trials, 8 to 26 weeks. For post-COVID applications, duration is typically determined by clinical response and may extend to months. I reassess at 12-week intervals.

The Nattokinase-Serrapeptase Combination

The combination of nattokinase with serrapeptase (a proteolytic enzyme derived from Serratia marcescens bacteria) is widely discussed in the post-COVID community and included in several practitioner protocols.

The rationale: Serrapeptase has anti-inflammatory and fibrinolytic properties that may complement nattokinase’s action. Serrapeptase preferentially degrades non-living tissue (biofilm, fibrin, inflammatory mediators) while sparing living tissue.

The evidence: Clinical evidence for this specific combination is minimal. Individual studies exist for each enzyme separately, but the synergistic effect is theoretical rather than demonstrated. One small study (Bhatt et al., 2020) showed improved inflammatory markers with the combination in dental patients, but this does not extrapolate directly to post-COVID microclot applications.

My approach: I am open to the combination for patients who tolerate it, particularly those with post-COVID symptoms. Typical combined dosing is nattokinase 2,000-4,000 FU plus serrapeptase 120,000-240,000 SPU daily. But I am honest that the evidence for synergy is theoretical.

Some practitioners also add bromelain (a pineapple-derived protease with anti-inflammatory and fibrinolytic properties) to create a “triple enzyme” protocol. Again, mechanistically interesting but clinically unproven as a combination.

Quality and Product Selection

FU standardization. Nattokinase potency should be measured in fibrinolytic units (FU), not milligrams. A product labeled “100mg nattokinase” without FU specification is inadequate — the enzymatic activity per milligram varies by production method and preparation.

Vitamin K2 content. Natto (the food) is a significant source of vitamin K2 (menaquinone-7, or MK-7). Some nattokinase supplements retain vitamin K2, others remove it. This matters clinically: vitamin K2 promotes coagulation (the opposite of nattokinase’s fibrinolytic effect) and interacts with warfarin. If you are taking nattokinase for fibrinolytic purposes, a vitamin K2-free preparation is generally preferable. Check the label.

Third-party testing. As with any supplement, look for products with independent third-party testing for identity, potency (FU verification), and contaminants.

NSP-2 strain. Products using the Bacillus subtilis natto NSP-2 strain, which is the most studied for nattokinase production, offer the closest match to the clinical trial evidence.

Safety and Considerations

Bleeding Risk: The Primary Concern

Let me be direct about this. Nattokinase is not a benign supplement in the way that, say, magnesium is. It has genuine pharmacological activity that affects hemostasis (the blood clotting system). This means it carries genuine bleeding risk.

Absolute contraindications:

• Active bleeding of any kind
• Hemorrhagic stroke history — nattokinase could increase the risk of recurrence
• Severe thrombocytopenia (very low platelet count) — further impairment of hemostasis is dangerous
• Planned surgery within 2 weeks — discontinue nattokinase at least 14 days before any planned surgical procedure

Drug interactions requiring physician oversight (do not combine without medical supervision):

• Warfarin — nattokinase potentiates anticoagulant effect. There are case reports of supratherapeutic INR and bleeding in patients combining nattokinase with warfarin
• Direct oral anticoagulants (DOACs) — apixaban, rivarelbivaban, dabigatran, edoxaban. The combination increases bleeding risk
• Antiplatelet agents — aspirin, clopidogrel, ticagrelor. Additive effect on hemostasis
• Heparin and low molecular weight heparins — contraindicated in combination
• NSAIDs (ibuprofen, naproxen) — these also affect hemostasis and the combination increases GI bleeding risk

Relative cautions:

• History of GI bleeding or peptic ulcer — increased risk
• Heavy menstruation — may worsen menstrual bleeding
• Uncontrolled hypertension — while nattokinase lowers blood pressure modestly, severe uncontrolled hypertension increases bleeding risk from any anticoagulant or fibrinolytic
• Pregnancy — insufficient safety data; avoid

Other Side Effects

At standard doses, nattokinase is generally well-tolerated. Reported side effects in clinical trials include:

• Mild gastrointestinal discomfort (uncommon)
• Easy bruising (dose-related, reflects anticoagulant activity)
• Prolonged bleeding from minor cuts (reflects anticoagulant activity)

Monitoring

For patients taking nattokinase at therapeutic doses, particularly those in post-COVID protocols, I recommend:

• Baseline coagulation assessment — at minimum, PT/INR and a complete blood count
• D-dimer — a marker of fibrin degradation that can help assess whether fibrinolytic activity is occurring (though interpretation is nuanced)
• Follow-up labs at 4 to 8 weeks — repeat coagulation parameters and D-dimer
• Report unusual bruising or bleeding immediately to your physician

What I Tell My Patients

Nattokinase is a real fibrinolytic enzyme with demonstrable pharmacological activity. It is not snake oil, and it is not a miracle cure. For post-COVID patients with suspected microclot pathology, it represents a mechanistically rational intervention that is being used clinically while we await the controlled trial data we need.

In our clinical experience at St. George Hospital, nattokinase is one component of a comprehensive approach to post-COVID recovery that includes therapeutic apheresis, whole-body hyperthermia, targeted supplementation, and ongoing monitoring. It is not a standalone treatment for long COVID, and anyone selling it as such is doing patients a disservice.

If you are considering nattokinase for post-COVID symptoms, work with a physician. Get baseline labs. Understand the bleeding risks. Do not combine it with blood thinners without medical supervision. And maintain realistic expectations: it may help, it may not, and we are being honest about the current limits of the evidence.

The Bottom Line

Nattokinase has moderate evidence supporting fibrinolytic activity and modest blood pressure reduction in humans. Its application to post-COVID microclot pathology is mechanistically sound but clinically unproven — an important distinction that I refuse to blur. At standard doses (2,000 to 4,000 FU daily), it has a reasonable safety profile for patients who are not on anticoagulant therapy and do not have bleeding risk factors. It carries real bleeding risk that demands respect. Use it with medical oversight, not based on internet protocols alone.

References

1. Sumi H, et al. A novel fibrinolytic enzyme (nattokinase) in the vegetable cheese natto; a typical and popular soybean food in the Japanese diet. Experientia. 1987;43(10):1110-1111.
2. Kurosawa Y, et al. A single-dose of oral nattokinase potentiates thrombolysis and anti-coagulation profiles. Sci Rep. 2015;5:11601. doi:10.1038/srep11601
3. Kim JY, et al. Effects of nattokinase on blood pressure: A randomized, controlled trial. Hypertens Res. 2008;31(8):1583-1588. doi:10.1291/hypres.31.1583
4. Ren NN, et al. A clinical study on the effect of nattokinase on carotid artery atherosclerosis and hyperlipidemia. Zhonghua Yi Xue Za Zhi. 2017;97(26):2038-2042.
5. Pretorius E, et al. Persistent clotting protein pathology in Long COVID/Post-Acute Sequelae of COVID-19 (PASC) is accompanied by increased levels of antiplasmin. Cardiovasc Diabetol. 2021;20:172. doi:10.1186/s12933-021-01359-7
6. Kell DB, et al. A central role for amyloid fibrin microclots in long COVID/PASC: Origins and therapeutic implications. Biochem J. 2022;479(4):537-559.
7. Pais E, et al. Effects of nattokinase, a pro-fibrinolytic enzyme, on red blood cell aggregation and whole blood viscosity. Clin Hemorheol Microcirc. 2006;35(1-2):139-142.
8. Hsia CH, et al. Nattokinase decreases plasma levels of fibrinogen, factor VII, and factor VIII in human subjects. Nutr Res. 2009;29(3):190-196.
9. Fujita M, et al. Thrombolytic effect of nattokinase on a chemically induced thrombosis model in rat. Biol Pharm Bull. 1995;18(10):1387-1391.
10. Jaeger B, et al. Impaired microcirculation and endothelial dysfunction in post-COVID syndrome: Clinical observations and therapeutic implications. [Research in progress, St. George Hospital, Bad Aibling.]