Nattokinase seems to alter blood coagulation chemistry, and there’s a concern it may increase the risk of bleeding. In people with mildly elevated cholesterol levels, taking 2,000 fibrinolytic units (FU) of nattokinase daily for 8 weeks prolonged the aPTT (activated partial thromboplastin time — a measure of how long it takes blood to clot).[1]
However, there have been only 2 reported cases of bleeding with nattokinase in older adults with cardiovascular conditions. In one case, 400 mg of nattokinase was taken daily for 7 days with aspirin (a blood thinner).[2] In the other case, an unknown, likely large dose of nattokinase (sometimes a “handful” of capsules) was taken daily.[3]