Healthy cannabis users, ages 18–45, were invited to reside on the secure clinical research unit of the National Institute on Drug Abuse (NIDA) Intramural Research Program, National Institutes of Health during 24 h monitored cannabis abstinence. Participants self-reported cannabis dependence or abuse, and had a positive urine cannabinoid test (50 ng/mL) that supported a history of cannabis exposure. The NIDA Institutional Review Board approved the study. All participants provided written informed consent and were financially compensated for time and inconvenience. Before inclusion, each participant underwent thorough medical (physical exam, ECG, blood, and urine chemistries) and psychological evaluations, including past and recent drug use history. All subjects in this study were required to have normal serum creatinine (0.6–1.2 mg/dL), and have no history or symptoms of renal disease in order to participate. Thus, any changes that occurred in urine creatinine were the result of changes in hydration rather than altered kidney function and/or diet. Twenty-four hour medical surveillance prevented access to unauthorized licit or illicit drugs. In addition, random urine drug tests for amphetamines, cannabinoids, cocaine, opiates, and phencyclidine were performed. All individual urine voids were collected ad libitum for up to 30 days.