Cannabis use disorders are an important public health problem in the United States, but there are no effective medications available to treat these disorders. The investigators intend to test a medication with interesting properties, nabilone, as a treatment for cannabis dependence and to study the relationship of this treatment with the brain using functional MRI brain scans. Nabilone and marijuana have similar effects upon behaviors and the human body, suggesting that nabilone may decrease cannabis withdrawal symptoms while allowing treatment-seeking patients to benefit from behavioral treatments when they are trying to stop using cannabis. The investigators propose to assess the relationship of nabilone, when added to behavioral treatment, on cannabis use patterns in cannabis-dependent patients. The investigators also aim to determine the effects of nabilone on performance on neuropsychological tests and to assess the correlation of neuropsychological performance to brain changes using functional MRI brain scans. The investigators hypothesize that patients receiving nabilone will reduce their use of cannabis more than patients receiving placebo during this 10-week treatment trial.