Nattokinase is an enzyme extracted from natto, a traditional Japanese fermented soybean food. It has been studied primarily for its fibrinolytic (clot-dissolving) activity, which has led to interest in its potential use for supporting stroke prevention and recovery, particularly ischemic strokes caused by blood clots. Preclinical studies and some small human trials suggest that nattokinase can degrade fibrin, reduce blood clot formation, and potentially lower blood pressure. For instance, several animal and in vitro studies have demonstrated its ability to dissolve thrombi and improve blood flow.

However, robust clinical evidence in humans is limited. A few small-scale clinical trials have reported reductions in blood pressure and improvements in blood flow markers among subjects using nattokinase, but there is a lack of large, well-controlled studies directly demonstrating a reduction in stroke incidence or improved outcomes post-stroke. Most available human data focus on surrogate markers (like blood pressure and D-dimer levels) rather than direct stroke outcomes. Furthermore, professional guidelines do not currently recommend nattokinase for stroke prevention or treatment due to insufficient evidence and unclear safety profiles, especially when combined with conventional anticoagulant or antiplatelet medications.

In summary, while nattokinase has a plausible biological mechanism and some preliminary supporting data, the overall evidence for its use in stroke is weak (rated 2/5), and well-designed clinical trials are needed before it can be considered a validated treatment or preventive strategy.