ELIGIBILITY: Although there was no change to the original eligibility criteria, for the remainder of the trial we limited recruitment to patients who were within 3 hours of injury. PRIMARY OUTCOME: The primary outcome included only patients randomised within 3 hours of injury. The primary outcome is death in hospital within 28 days of injury among patients randomised within 3 hours of injury (cause-specific mortality was also recorded). SAMPLE SIZE A study with 10,000 traumatic brain injury (TBI) patients randomised within 3 hours of injury would have about 90% power (two sided alpha=1%) to detect a 15% relative reduction (from 20% to 17%) in all‐cause mortality. About three thousand patients had been recruited beyond three hours of injury already, therefore the total sample size was increased to approximately 13,000 patients. STATISTICAL ANALYSIS: We expected tranexamic acid (TXA) to be most effective when given soon after injury, when tissue plasminogen activator (TPA) levels are highest, and less effective when given several hours after injury when the risk of thrombotic DIC may be increased. We planned to examine this hypothesis by conducting a subgroup analysis of the effect of TXA according to the time interval between injury and TXA treatment (≤1, > 1 to ≤ 3, > 3 h). The outcome measure for this subgroup analysis was death due to head injury. RATIONALE: After the CRASH‐3 trial had started, new research suggested that TXA was likely to be most effective in the first few hours after injury and less effective when given later. To ensure that the CRASH‐3 trial was large enough to reliably confirm or refute an early (<3 hours) treatment benefit, the sample size was increased from 10,000 to 13,000 patients with the aim to enrol 10,000 patients within 3 hours of injury. In addition, the primary outcome was been amended to deaths among patients treated within 3 hours of injury.