Thrombosis is an adverse event of special interest with marstacimab due to the drug’s mechanism of action as a rebalancing agent that targets tissue factor pathway inhibitor (TFPI) to increase thrombin generation for clot formation. Thrombotic events have been observed with other nonfactor hemophilia therapies. The use of marstacimab should include close monitoring for symptoms of thrombosis and other adverse events, especially in surgical settings in combination with clotting factor concentrates or bypassing agents.
Marstacimab was approved by the U.S. Food and Drug Administration (FDA) in October 2024 and the European Medicines Agency (EMA) in November 2024 for routine prophylaxis in adults and adolescents aged 12 years and older with severe hemophilia A or B without inhibitors. It is currently approved in the United States, 27 EU member states, Iceland, Liechtenstein, Norway, UK, Switzerland, Canada, Australia, and Singapore.
The WFH and NBDF are monitoring developments and will update the community as more information becomes available.
See Pfizer’s letter on the marstacimab adverse event issued December 22, 2025, here. To see a PDF version of this statement, please click here.










