Does Nattokinase Cause Constipation?

Across three years of the largest placebo-controlled nattokinase trial to date, no cases of constipation were recorded as an adverse event in either the treatment arm or the placebo arm.

That silence in the data matters. Chronic constipation affects 9–20% of American adults [7] at any given time, and nattokinase is now used widely for cardiovascular support — so the two overlap by coincidence more often than most people realize. This article walks through what the clinical evidence actually shows about nattokinase and constipation, what side effects the studies do document, and what to check first if your bowels changed after starting nattokinase.

Table of Contents

1. The Short Answer: What Clinical Trials Show
2. Digestive Side Effects Nattokinase Does Cause
3. What's Inside a Nattokinase Capsule — And Why It's Not a Constipation Risk
4. The Real Reasons You Might Feel Constipated After Starting Nattokinase
5. Natto and the Gut: What the Fermented Soy Evidence Suggests
6. When to Stop and Talk to Your Doctor
7. BioAbsorb Nattokinase — A Clean-Excipient Formulation
8. Frequently Asked Questions
9. Conclusion

1. The Short Answer: What Clinical Trials Show

The direct answer to "does nattokinase cause constipation" is that clinical trials have not identified it as a side effect. The most rigorous data comes from the Nattokinase Atherothrombotic Prevention Study (NAPS) [1] — 265 participants with a median age of 65.3 randomized to 2,000 FU/day of nattokinase or placebo for three years. No significant differences were reported in adverse events between the two groups, and constipation did not appear in the detailed gastrointestinal adverse event list [3] for either arm.

A separate 2023 systematic review and meta-analysis of 6 RCTs (546 participants) [2] reached the same conclusion — no notable adverse events in any nattokinase or placebo group, with compliance rates exceeding 95%. And a real-world safety study of patients with vascular diseases [5] receiving 100 mg/day of nattokinase reported no adverse drug reactions and no drug interactions during follow-up.

Across the peer-reviewed record — controlled trials, meta-analyses, and real-world data combined — constipation is not a documented side effect of nattokinase supplementation. That doesn't mean an individual can't experience it, but it means the pattern isn't there in the population data.

2. Digestive Side Effects Nattokinase Does Cause

To understand why constipation isn't on the list, it helps to know what is. The digestive side effects most consistently reported across trials — when any are reported at all — sit at the opposite end of the spectrum. The NAPS trial's detailed adverse event breakdown showed diarrhea at 0.76% of nattokinase participants vs 0% of placebo, dyspepsia at 0.76% vs 2.26% (higher in placebo), and abdominal distension at 0% vs 0.75% [3] — again, higher in placebo.

A broader research review of nattokinase in blood-lipid contexts describes "occasional minor gastrointestinal discomfort, such as bloating, belching, etc." at low frequency in healthy people on long-term use — and notes that these usually resolve after dose or timing adjustment. The Memorial Sloan Kettering integrative medicine monograph [8] and the Drugs.com professional monograph [9] both list bleeding-related risks — not constipation — as the primary safety consideration, particularly with concurrent anticoagulant or antiplatelet medication.

The clinical picture, then, is a mild-diarrhea or mild-bloating profile at low rates statistically indistinguishable from placebo — not a constipation profile. This is worth keeping in mind if you're troubleshooting: the direction of the effect matters.

3. What's Inside a Nattokinase Capsule — And Why It's Not a Constipation Risk

Constipation from a supplement usually traces to one of two culprits: the active ingredient itself, or a filler with a well-known GI effect. Neither category applies to a standard nattokinase capsule. The active ingredient is a serine-protease enzyme, delivered at roughly 100 mg (2,000 FU) per capsule — the dose the European Food Safety Authority reports no adverse effects for [11], and one that has been safely taken at 540 mg (10,800 FU) daily for 12 months in human studies. Enzymes at this dose do not physically bulk stool or dehydrate it.

The excipients in a well-formulated nattokinase capsule are also inert with respect to bowel motility. A typical ingredient list — hypromellose (the veggie capsule shell), microcrystalline cellulose, magnesium stearate, and gellan gum — contains no iron, no calcium, and no aluminium. That matters because, per Parkview Health's patient education review [10], iron supplements are one of the most common causes of supplement-induced constipation, and calcium supplements — especially calcium carbonate — also contribute at higher doses.

If you're taking a multi-ingredient cardiovascular formula that combines nattokinase with calcium, iron, or a calcium-carbonate antacid base, the constipation lever may be one of those companions rather than the nattokinase itself. Reading the full label — not just the front — is the fastest way to rule this out.

4. The Real Reasons You Might Feel Constipated After Starting Nattokinase

Timing coincidence is the honest first suspect. Chronic idiopathic constipation affects between 9% and 20% of US adults [7], and in a national survey of 4,702 people with recent constipation, roughly 24% met Rome IV diagnostic criteria for the chronic form. Given that nattokinase is most often started by adults aged 45+ — the same demographic where chronic constipation prevalence climbs — some overlap is statistically expected without any causal link.

Second, other changes typically travel with a "new supplement" decision. Someone starting nattokinase may also be reviewing their diet, drinking less coffee, adding a magnesium-free multivitamin, or taking a new blood-pressure medication — several classes of which include constipation as a documented side effect. When multiple variables move at once, it's easy to attribute the change to whichever one is newest, but the actual driver may be elsewhere in the stack.

Third, hydration. Nattokinase is typically taken on an empty stomach for absorption, and empty-stomach dosing routines can crowd out morning fluid intake if not planned around them. This is easy to fix — pair the capsule with a full glass of water — but it's a plausible practical reason someone might notice slower bowels in the first days on a new supplement.

5. Natto and the Gut: What the Fermented Soy Evidence Suggests

Nattokinase is produced by Bacillus subtilis during the fermentation of soybeans into natto. Both the parent food and the source organism point in the opposite direction from constipation. A 2025 randomized, double-blind, placebo-controlled trial of 30 healthy adults [4] gave 3 grams of natto powder daily for 4 weeks and found significant improvement in defecation-related discomfort in the test group versus placebo, along with changes in stool color and odor consistent with gut-flora shifts.

The B. subtilis story is even more direct. A 2025 meta-analysis of RCTs in Frontiers in Pharmacology [6] found that live combined B. subtilis and Enterococcus faecium significantly improved constipation outcomes — via restoration of intestinal microbial balance, promotion of short-chain fatty acid production, and stimulation of peristalsis. Probiotic strains of the same species that produces nattokinase are used clinically to relieve constipation — the source organism does not point toward causing it.

Nattokinase supplements are not natto and are not probiotics — the enzyme is purified from the fermentation broth. So these food and probiotic findings don't automatically transfer to a capsule. But they do explain why the constipation hypothesis has no mechanistic support from either the source food or the source organism.

6. When to Stop and Talk to Your Doctor

Constipation on its own is rarely a reason to stop nattokinase — the more common step is to check the rest of the routine (hydration, fiber, magnesium intake, other supplements and medications) for a likelier cause. But there are signs that warrant a call to your healthcare provider before the next dose.

The Drugs.com professional monograph [9] flags a small number of conditions where nattokinase should be avoided entirely: active peptic ulcer, ischemic stroke history, coagulation disorders, and the pre- and post-operative window. If any of these apply to you, digestive symptoms are less important than the underlying bleeding-risk conversation with your doctor. Similarly, any sign of unusual bleeding — blood in stool, unusual bruising, prolonged bleeding from small cuts, or heavy nosebleeds — is a reason to stop and check in promptly, regardless of bowel changes.

For persistent constipation that doesn't resolve within a week of adjusting fluids, fiber, and companion supplements, a healthcare provider can help distinguish supplement-related from unrelated causes. This is especially worth doing after age 50, or if constipation is a new pattern rather than a familiar one.

7. BioAbsorb Nattokinase — A Clean-Excipient Formulation

For readers evaluating a nattokinase supplement with digestive comfort in mind, formulation and excipient choice are worth reading closely. BioAbsorb Nattokinase Enzyme delivers 100 mg (2,000 FU activity) of nattokinase per capsule — the same dose used across the majority of published clinical trials — in a DRcaps delayed-release veggie capsule that protects the enzyme from stomach acid without using the phthalates and plasticizers found in some conventional enteric coatings.

The non-medicinal ingredient list is deliberately short — hypromellose, microcrystalline cellulose, magnesium stearate, and gellan gum — with no added iron, no calcium carbonate, no vitamin K2, and no soy oil carriers. Because iron and calcium carbonate are among the most common supplement-linked constipation triggers, keeping them off the ingredient list is a meaningful choice for readers who monitor digestive comfort.

Each batch of BioAbsorb Nattokinase is manufactured in a Canadian GMP-certified facility and third-party tested for potency (≥2,000 FU per capsule), heavy metals, gluten, and microbial contaminants. The 180-day supply option costs roughly $0.28 per day — a strong cost-per-day position for a one-capsule-daily protocol. Take one capsule daily on an empty stomach, ideally with a full glass of water.

Frequently Asked Questions

Is constipation a listed side effect of nattokinase?

No. Neither the NAPS trial adverse event record [3] nor the 2023 systematic review of 6 RCTs identifies constipation as a nattokinase side effect. The digestive effects that have been reported at low rates are mild diarrhea, bloating, and dyspepsia — not constipation.

What are the most common side effects of nattokinase?

In clinical trials, nattokinase is very well tolerated at standard 2,000 FU doses. When any side effects are reported, they are typically mild gastrointestinal effects like bloating, belching, or occasional nausea, at rates comparable to placebo. The clinically important safety concern per Memorial Sloan Kettering [8] is increased bleeding risk when combined with anticoagulant or antiplatelet medications — not digestive effects. For a fuller treatment of that interaction, see nattokinase as a natural blood thinner.

Should I stop taking nattokinase if I become constipated?

Not automatically. Given 9–20% background prevalence of chronic constipation [7] in US adults, the change may not be caused by the supplement. Start by reviewing hydration, fiber, magnesium intake, and any concurrent iron or calcium supplements, medications, or diet changes. If constipation persists more than a week after adjustment, or you notice any bleeding symptoms, contact your healthcare provider.

Does taking nattokinase on an empty stomach cause digestive problems?

Empty-stomach dosing is standard because it improves enzyme survival, and clinical trials use this protocol without reporting increased GI effects. A minority of users report mild nausea or bloating on an empty stomach; pairing the capsule with a full glass of water and moving it to first thing in the morning usually resolves this. There is no evidence that empty-stomach dosing causes constipation specifically.

Can natto (the food) cause constipation?

No — the evidence points the other way. A 2025 RCT of 30 healthy adults [4] found that 3 grams of natto powder daily for 4 weeks improved defecation comfort compared to placebo. Fermented soybean foods are traditionally consumed for digestive support, not avoided for constipation risk.

Which supplements are more likely to cause constipation than nattokinase?

Per patient-education reviews from institutions like Parkview Health [10], iron supplements (especially ferrous sulfate) and calcium supplements (especially calcium carbonate) are the two supplement categories most consistently linked to constipation. If you take a stacked cardiovascular formula, check whether either is in the ingredient list before blaming the nattokinase.

Conclusion

The current clinical evidence — a 3-year 265-participant RCT [1], a meta-analysis of 6 RCTs covering 546 participants, and real-world safety data — does not link nattokinase to constipation. When digestive side effects do occur, they lean toward mild diarrhea or bloating at rates comparable to placebo, not the opposite. If you're looking for a clean nattokinase formulation without the excipients most often tied to supplement-induced constipation, BioAbsorb Nattokinase delivers 2,000 FU per capsule with no iron or calcium fillers and third-party batch testing for potency.

For broader safety context, see the full nattokinase side effects overview, or read the nattokinase dosage guide for timing and dose-adjustment guidance. For the wider picture on how nattokinase works and what the cardiovascular research shows, see the complete guide to nattokinase.

Research References

1. Nattokinase atherothrombotic prevention study: A randomized controlled trial (NAPS). Clinical Hemorheology and Microcirculation, Vol. 78, Issue 4 (2021). Three-year placebo-controlled RCT in 265 participants finding no significant difference in adverse events between 2,000 FU/day nattokinase and placebo; primary source for the constipation-not-reported finding.
2. Nattokinase Supplementation and Cardiovascular Risk Factors: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Reviews in Cardiovascular Medicine, Vol. 24, Issue 8 (2023). Meta-analysis of 6 RCTs (546 participants) reporting no notable adverse events in any nattokinase or placebo group.
3. NAPS trial — Adverse Events Record (NCT02080520). ClinicalTrials.gov (results record, 2021). Provides itemized GI adverse event data by category showing diarrhea, dyspepsia, and abdominal distension were the reported GI events — constipation was not among them in either arm.
4. Improvement of Bowel Movement in Healthy Adults Through Intake of Natto Powder For 4 Weeks — A Randomized, Placebo-Controlled, Double-Blind, Clinical Study. Food and Nutrition Journal, Vol. 10, Article 329 (2025). RCT of 30 adults showing significant improvement in defecation-related discomfort with 3g/day natto powder vs placebo.
5. Data Recorded in Real Life Support the Safety of Nattokinase in Patients with Vascular Diseases. Nutrients, Vol. 13, Issue 6, article 2031 (2021). Real-world observational study reporting no adverse drug reactions or drug interactions during follow-up of vascular-disease patients on 100 mg/day nattokinase.
6. Efficacy and safety of live combined Bacillus subtilis and Enterococcus faecium in patients with constipation: a meta-analysis of randomized controlled trials. Frontiers in Pharmacology (2025). Demonstrates that probiotic strains of the source species of nattokinase (B. subtilis) are used clinically to improve — not cause — constipation.
7. Chronic Constipation in the United States: Results From a Population-Based Survey. American Journal of Gastroenterology, Vol. 115, Issue 6 (2020). Provides the 9–20% chronic constipation prevalence figure used to frame background risk.
8. Nattokinase — About Herbs, Botanicals & Other Products. Memorial Sloan Kettering Cancer Center Integrative Medicine. Institutional monograph identifying bleeding-risk drug interactions — not GI effects — as the primary safety concern for nattokinase.
9. Nattokinase — Professional Monograph. Drugs.com Natural Products Professional Reference (2026). Clinician-facing reference summarizing dosing, safety, and contraindications for nattokinase.
10. Why some vitamins and medications can cause constipation. Parkview Health (2026). Patient-education reference identifying iron and calcium supplements as the leading supplement causes of constipation.
11. Nattokinase benefits, dosage, and side effects. Examine.com. Independent research reference citing the European Food Safety Authority's no-adverse-effect assessment at 100 mg/day (2,000 FU).

About the Author

David Kimbell is a health writer, digital entrepreneur and former aerospace engineer, based in Ottawa, Canada. He loves translating complex science into clear, actionable guidance for consumers seeking evidence-based solutions.

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Important Disclaimers

Medical Disclaimer: This article provides educational information only and is not intended as medical advice. Always consult with a qualified healthcare provider before starting any new supplement, especially if you have existing health conditions, take medications, or are pregnant or nursing.

FDA/Health Canada Statement: These statements have not been evaluated by the Food and Drug Administration or Health Canada. This product is not intended to diagnose, treat, cure, or prevent any disease.