Nattokinase has been used as a supplement for nearly 20 years with no known side effects. In fact, Nattokinase has demonstrated most of the benefits of pharmaceutical agents designed to restrict blood clotting ( such as Warafin, Herapin and t-PA) without any of the side effects of those medications. In addition, it has not been recognized to cause any allergic reactions There have never been any reports of toxicity associated with Nattokinase supplementation.