 A total of 20 subjects will participate in this four week, between groups, double-blind, placebo controlled study. Subjects will participate in two experimental sessions separated by approximately one week. Subjects will be randomized to receive either 50 mg cycloserine or placebo combined with cue exposure. Several physiological and subjective outcome measures e.g., heart rate, blood pressure, galvanic skin response will be obtained during the sessions. Experimental sessions will last approximately 4.5 hours with follow-up sessions lasting approximately thirty minutes. Our aims are: 