Nattokinase is a fibrinolytic enzyme extracted from natto, a traditional Japanese fermented soybean dish. Its use in supporting or treating arteriosclerosis (the thickening and hardening of arterial walls) is primarily justified by scientific interest and some early clinical research, rather than by longstanding traditional use for this specific condition. Nattokinase has been shown in in vitro and animal studies to enhance fibrinolysis, thereby reducing blood clot formation and potentially improving blood flow. Some small human trials and observational studies have suggested that nattokinase supplementation may help lower blood pressure, decrease plasma fibrinogen, and improve lipid profiles—all factors that contribute to arteriosclerosis risk. However, the evidence base is limited: most studies are small, short-term, and sometimes methodologically weak. There are no large-scale, long-term clinical trials definitively proving that nattokinase prevents or reverses arteriosclerosis in humans. Safety concerns also remain, especially regarding its use alongside anticoagulant medications. In summary, while there is a plausible biological rationale and some early clinical evidence supporting nattokinase’s potential benefits for cardiovascular health and arteriosclerosis, the evidence is currently modest (rated 2/5), and more robust clinical research is needed to confirm its efficacy and safety for this purpose.