The study consists of twelve weeks of open-label dronabinol. All eligible study subjects will be started on open-label dronabinol 2.5mg/day for 3 weeks. The dose will be increased to 5mg/day at visit 2 (Week 3), to10mg/day at visit 3 (Week 6), and to 15mg/day at Visit 4 (Week 9) unless clinical improvement is attained at a lower dose (clinical improvement will be assessed by the investigator with respect to obsessive thoughts, urges and compulsive behaviors). Subjects will be seen every three weeks during the 12-week, open-label study. All efficacy and safety assessments will be performed at each visit. Subjects who are not compliant with their use of study medication (i.e. failing to take medication for three or more consecutive days) will be discontinued from the study.