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.

Inclusion Criteria:

• Adults 18-70 years
• Evidence of acute Post Traumatic Event (PTE) exposure defined by Diagnostic and Statistical Manual criterion 'A'
• Evidence of a significant stress response defined by DSM V PTSD criterion 'B' and meeting at least criteria for 'partial PTSD' or a minimum of 3 of 4 DSM V PTSD symptom criterion
• Score of 60 or higher on the Subjective Units of Distress Scale (SUDS)
• Living in New York tri-state area
• Fluency in English, Spanish

Exclusion Criteria:

• Admission to an intensive care unit or other overnight admission
• In the Emergency Department for more than 6 hours
• Evidence of ongoing traumatic exposure (e.g. domestic violence)
• Evidence of psychotic symptoms
• Evidence of homicidality/suicidality
• Adults with an open head injury, a positive CT scan, loss of consciousness >30 seconds or survivors in a coma
• Adults in police custody or Department of Correction (DOC) patients
• Inability to understand the study's procedures, risks, or side effects, or otherwise unable to give informed consent.
• Females who are nursing or pregnant (as confirmed by a positive urine pregnancy test).
• Permanent cardiac pacer implant.
• . Self-reported medical conditions that may be affected by DEX including asthma, epilepsy,diabetes, liver disease, kidney disease, thyroid disorder, muscle disorder, history of malaria,tuberculosis, osteoporosis, glaucoma or cataracts.
• Self-reported adverse reactions to steroids