To determine if a single dose of dexamethasone (5 mg) administered in the first 12 hours following a potentially traumatic event alters a) cortisol and FKBP5 RNA the next day in the periphery measured in saliva; b) FKBP5 methylation by 1 month; c) executive functioning and emotion regulation functioning; d) psychophysiological (heart rate, respiration, skin conductance) in response cued reminders of the trauma; e) enhances the likelihood of remission of PTSD symptom severity.