Bleeding causes most early deaths from major trauma, often exacerbated by abnormalities in blood clotting. Tranexamic acid (TXA) reduces abnormal clot breakdown. There is some evidence from lower income countries that early prophylactic use of TXA improves outcomes. However as it is uncertain if the possible benefits outweigh the risks (dangerous blood clots) in advanced trauma care systems with other rapid and effective treatments to stop bleeding and improve blood clotting, TXA is rarely used prophylactically in NZ. In this international study, severely injured patients at risk of abnormal blood clotting will be randomly allocated to receive TXA or placebo commenced by ambulance staff at the scene and completed in hospital, in addition to all usual treatment. Patients will be followed for 6 months to determine if treatment with TXA improves survival and/or reduces disability. If effective, prophylactic TXA will become standard treatment in NZ ambulance services and Emergency Departments.