The majority of blood substitutes are hemoglobin‐based oxygen carriers (HBOCs) that utilize human (HbA) or bovine (bHb) hemoglobin.2 Examples of HBOCs include Hemopure® (polymerized bHb made by OPK Biotech),3 Hemospan® (PEGylated HbA made by Sangart),4 PolyHb‐SOD‐CAT (hemoglobin cross‐linked via glutaraldehyde with superoxide dismutase and catalase),5 and many others.6, 7, 8 Unfortunately, despite early promise, most HBOCs eventually failed Phase III clinical trials due to adverse reactions such as vasoconstriction, hypertension, and stroke. Here, CAT is linked to Stroke.