 Following an initial assessment evaluating eligibility for the study, eligible participants will be randomly assigned to one of two treatment groups: imaginal exposure IE plus DCS 100mg or IE plus placebo sugar pill. DCS is a broad spectrum antibiotic that has recently been implicated as a cognitive enhancer and may enhance the treatment that occurs. The participant, assessor and treating clinicians will be blinded to which pill the participant is receiving. The dose of medication will need to be taken only on the days of therapy sessions during which the exposure occurs approximately 9 times. Both groups will be treated with a standardized cognitive-behavioral exposure therapy protocol utilizing gold-standard treatment, consisting of 12-14 weekly individual one-on-one sessions with a highly qualified clinical psychologist. Treatment interventions include imaginal exposure, graduated in vivo exposure, psycho-education, relaxation training, behavioral activation, and cognitive restructuring. Assessments will occur prior to treatment, following sessions 3, 6 and 10, following completion of treatment, and 6 months after the conclusion of treatment. In addition, all participants will be genotyped once for the BDNF SNP Val66Met using a non-invasive saliva sample. 