Generally, nattokinase seems to be safe in adults when taken by mouth in doses of 100 mg (2,000 fibrinolytic units) daily.[1] Although this dose may reduce blood coagulability biomarkers (e.g., prolonged activated partial thromboplastin time (aPTT), a measure of how long it takes blood to clot),[2] there have been no reports of bleeding in adults without apparent health conditions even at doses up to 552 mg daily for four weeks.[3] However, in older adults with chronic conditions, there have been rare reports of bleeding.[4][5]
There have been reports of severe allergic reactions to nattokinase, including anaphylaxis, an itchy rash, and difficulty breathing. This can occur in people allergic to either polygamma glutamic acid (PGA, a sticky substance also found in jellyfish) or to nattokinase itself.[6] However, the European Commission concluded that the risk of allergic reactions with nattokinase is no higher than the risk with soy products.[1]