Cannabis use is a significant public health concern that disproportionately affect youth. Although promising psychosocial interventions are being developed, most youth do not benefit from these interventions alone. Given the clinical demand for effective treatments, the National Institute on Drug Abuse (NIDA) identified the critical need for data on the tolerability and potential efficacy of medications in adolescents. The purpose of this two-year study is to test if and how topiramate, a medication under intense study for treating several drugs of abuse, reduces cannabis use among teenagers. To this end, the investigators will randomize 56 nontreatment-seeking regular cannabis users (15 or 20 years old) to receive topiramate or placebo for 6 weeks. Youth will monitor their cannabis use for the 6-week period using handheld electronic diaries and complete assessments of reactivity to cannabis-related cues.