The purpose of this study is to determine if certain chemicals in the body, called endocannabinoids, are affected by heavy marijuana use or marijuana withdrawal. Participation lasts approximately five weeks. In the first week, participants can use marijuana as usual and will complete mobile assessments asking about their current feelings four times each day for seven days. Participants will also complete three 5-hour laboratory visits during this week to assess levels of endocannabinoids in the blood at different times of day. One month after the first laboratory visit, participants will complete the same tasks (three laboratory visits and daily mobile assessments) while maintaining abstinence from marijuana for one week.