This study will examine sex differences in how the brain processes reward in people that regularly use cannabis. Men and women that use cannabis at least 4 days a week will participate in a study involving 3 in-person visits. The first visit will be used to determine eligibility for the study. The other two visits will last 7 hours each and take place at least two days apart. Visits include administration of either THC, the chemical responsible for many of the psychoactive effects of cannabis, or a placebo; completion of questionnaires; and a 1.5-hour brain scan.