Background: Mechanical circulatory support (MCS) causes high levels of shear stress and complications such as hemolysis, von Willebrand factor (vWF) degradation and gastrointestinal (GI) bleeding can arise. Shear stress unfolds vWF, allowing for cleavage by ADAMTS13. Doxycycline is a drug that inhibits ADAMTS13, however, the current clinical dose may not protect vWF. During hemolysis, hemoglobin (Hb) binds to vWF and protects from ADAMTS13. Thus, this study aimed to assess how hemolysis impacts the efficacy of doxycycline to prevent vWF degradation. Methods: Human blood components were treated with Hb and doxycycline and subjected to VAD-like shear stress. vWF structure was first assessed using immunoblotting. vWF function was assessed by measuring ristocetin-induced platelet aggregometry and collagen binding activity. Results: In plasma, the presence of Hb or doxycycline alone restores vWF structure and function, suggesting protective effects. Paradoxically, the presence of doxycycline and Hb together reduced vWF structure and function. Whole blood treated with doxycycline reduced vWF structure and function in the presence of shear-induced hemolysis. Conclusion: Administration of doxycycline in MCS patients with hemolysis may prove ineffective, which could explain the discrepancies in vWF degradation observed in clinic, for treatment of GI bleeding. As doxycycline is seen to provide protective effects in the absence of Hb, a potentially negative interaction may be occurring between doxycycline and Hb. Thus, a higher dose of doxycycline may be feasible to overcome the negative effects of hemolysis. Furthermore, as hemolysis may result in Hb-mediated protection of vWF degradation, increased vWF activity may enhance vWFplatelet binding and elevate the risk of thrombosis.