Participation in the study takes 10 visits over a period of approximately two to three months. The first visit is a screening visit to determine if participants are eligible to participate. After the initial assessment visit, the weekly visits take about 30 minutes, with the exception of three therapy sessions which take approximately 60-90 minutes. The three therapy sessions will focus on participant's marijuana use and reasons they may have for stopping or cutting down on use. After completing an initial therapy session, participants will be randomly selected to receive the study medication, either vilazodone or placebo (a capsule that does not contain any active medication). Participants will have weekly study visits with a clinician. At each study visit, they will be asked to fill out forms and answer specific questions concerning their substance use, anxiety symptoms, and feelings in general. They will be providing urine samples to check for illegal drugs of abuse including marijuana and other drugs. They will also have blood samples drawn during to determine if you are taking the medication as directed.