The consumption of herbal supplements has become increasingly popular the United States. One such herbal supplement that is available at pharmacies and grocery stores is nattokinase. Nattokinase, a byproduct of soybean fermentation, may have some thrombolytic properties. We present the case of a patient who developed hemoperitoneum and subsequently passed away. We review the potential mechanisms of action of nattokinase and warn against consumption of herbal supplements, especially nattokinase.