Participation in this study will last 12 months, including all follow-up assessments. During the first study visit, participants will undergo baseline testing and be randomly assigned to receive either DBT or fluoxetine with clinical management. After a washout period, in which participants will transition off any medications they are currently taking, participants will receive 6 months of their assigned treatment. Participants receiving DBT will attend one 60-minute individual therapy session and one 90-minute group session every week. Participants assigned to the fluoxetine with clinical management condition will begin receiving 20 mg of fluoxetine daily and have their dose increased over the course of 4 weeks, based on tolerance, up to 40 mg. Participants assigned to fluoxetine may also be switched to citalopram, if the study psychiatrist thinks it will be more effective. Participants assigned to either fluoxetine or citalopram will undergo monthly blood tests to monitor the level of medication in their bodies.