Initially, there will be a dose escalation component within the progesterone group, but the differences in the level of circulating progesterone between the subgroups will be minimal compared to the control group. Hence, all progesterone treated patients are assumed to respond similarly to the treatment. (2) At the completion of the pilot phase at least 100 patients will be randomized into this study. GMH's trauma registry indicates that at least 145 potentially eligible TBI patients (age ≥ 18, GCS 4-12, blunt mechanism, and <11 hours from injury) will come to GMH during the study interval. Every effort will be made to obtain informed consent from a proxy in order to administer the study drug within the specified time limit (2 hours).