he study will begin with an inpatient phase (phase 1) of 5 days, during which abstinence is achieved, followed by a 4 week outpatient phase (phase 2). A single infusion of ketamine or midazolam will occur on day 3 of Phase 1. In addition to measures of mindfulness and impulsivity, stress sensitivity tests are incorporated into the design in order to elucidate mechanisms of action. The study hypotheses are: