Common (>10% users):

• No common side effects at 2000-4000 FU/day in most users.
• Mild GI complaints (nausea, soft stool) in <10%; usually resolves with food despite the empty-stomach optimization.

Less common (1-10%):

• Mild bruising or prolonged bleeding from minor cuts.
• Occasional nosebleeds (especially in dry climates / with prior nasal trauma).
• Headache (rare; usually mild).
• Allergic reaction (soy-derived production substrate — rare cross-reactivity even though purified nattokinase doesn't contain soy proteins; verify allergen-free product if soy-allergic).

Rare-serious (<1% but critical to flag):

• Cerebral hemorrhage / hemorrhagic stroke — case reports exist, particularly in patients combining nattokinase with anticoagulants (warfarin, DOACs) or antiplatelets (aspirin, clopidogrel). The most-cited case is a 52-year-old woman who developed cerebellar hemorrhage 7 days after starting nattokinase while on aspirin (Chang et al. 2008, Korean J Anesthesiol). Other case reports: GI bleed, hemarthrosis, retroperitoneal hematoma — all in patients on overlapping antithrombotics. Mechanism: additive fibrinolytic + antiplatelet effects, particularly in users with subclinical vascular fragility (small aneurysms, AVMs, hypertensive arteriopathy).
• Bleeding amplification with all antithrombotic drugs. Watch INR if on warfarin (INR can rise unpredictably). DOACs (apixaban, rivaroxaban, dabigatran) — combination not studied; theoretical and case-reported bleeding amplification → effectively contraindicated.
• Surgical bleeding — discontinue ≥2 weeks before any procedure with bleeding risk. Several case series document increased intraoperative bleeding when nattokinase wasn't held pre-op.
• Potential interaction with NSAIDs at high dose. Daily aspirin or chronic high-dose ibuprofen + nattokinase: additive antiplatelet / GI mucosal risk. Magnitude probably modest at typical OTC doses but flagged.
• Allergic reaction / anaphylaxis to soy-derived contaminants — rare; mostly mitigated in well-purified products.
• Theoretical: amyloid plaque destabilization — in vitro, nattokinase digests amyloid plaques (which has been claimed as a potential AD treatment). Speculative concern: in patients with cerebral amyloid angiopathy, plaque destabilization could theoretically increase microbleed risk. No clinical evidence either direction.

Specific watch periods:

• First 4 weeks: check BP weekly (home cuff), watch for bruising / bleeding gums, headache, focal neurological symptoms. Discontinue immediately if any signs of intracranial bleeding.
• 6-12 weeks: D-dimer, fibrinogen, lipid panel recheck if started for cardiovascular indication.
• Annually: CBC, INR (especially if any anticoagulant added), liver panel.
• Pre-procedure: discontinue ≥2 weeks before any surgery, dental work, biopsy, or contact-sport tournament.

Hard contraindications:

• Active anticoagulation (warfarin, DOAC, heparin).
• Active dual antiplatelet therapy (aspirin + clopidogrel post-stent).
• Active bleeding disorder (von Willebrand, hemophilia, thrombocytopenia).
• Recent hemorrhagic stroke or cerebral aneurysm.
• Pregnancy / breastfeeding (safety not established; theoretical concern about placental fibrin).
• Imminent surgery (within 2 weeks).