Baclofen Effects on Marijuana Dependence

Inclusion Criteria: * Physically healthy, as determined by a comprehensive physical examination and approval of the study physician, and mentally stable, as determined by a psychological evaluation by a licensed clinician, males or females who meet Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) criteria for marijuana dependence, ages 18-60. * Must be able to read. \[Subjects are required to be able to read because there are several self-administered measures that they must read, understand and provide written answers.\] * Females must be non-pregnant, non-lactating and either be of non-childbearing potential (i.e. sterilized via hysterectomy or bilateral tubal ligation or at least 1 year post-menopausal) or of child bearing potential but practicing a medically acceptable method of birth control. Examples of medically acceptable methods for this protocol include barrier (diaphragm or condom) with spermicide, an intrauterine device (IUD), the Nuvaring, oral contraceptives, levonorgestrel implant, hormonal injection or complete abstinence. * Intelligence quotient of ≥ 80. * Provide voluntary informed consent. Exclusion Criteria: * Participation in a clinical trial and receipt of investigational drug(s) during the previous 60 days. * Presence of magnetically active irremovable prosthetics, plates, pins, permanent retainer, bullets, etc. (unless a radiologist confirms that it's presence is unproblematic). An x-ray may be obtained to determine eligibility given the possibility of a foreign body. * Current use of non-removable medicated or nicotine skin patches. * Current or prior gambling problems (assessed by subject's self-report). * Current treatment for marijuana dependence. * Any other current drug dependence diagnoses (except nicotine dependence). * Any other psychiatric diagnoses. Current diagnosis/recent history (within past 6 months) of Major Depressive Disorder (MDD). * History of head trauma or injury causing loss of consciousness, lasting more than three (3) minutes or associated with skull fracture or inter-cranial bleeding or abnormal Magnetic Resonance Imaging (MRI). * Claustrophobia or other medical condition preventing subject from lying in the MRI for approximately one (1) hour. * Clinically significant cardiovascular, hematologic, hepatic (liver), renal (kidney), neurological, or endocrinological abnormalities, bloodwork, and/or electrocardiogram (EKG). * Uncontrolled diabetes or uncontrolled hypertension. * History of psychosis, seizures, or organic brain syndrome. * Vision problems that cannot be corrected with glasses. * Weight exceeding 300 pounds \[Imaging data acquisition is impaired with high weight individuals\]. * History of stroke. Case-by-Case: * Human immunodeficiency virus (HIV) positive on medication for symptoms. This will be determined on an individual basis by results from the physical examination and final approval by our study physician; all results from the physical within normal range for study inclusion. * Use of medications or natural herbs that cause sedation or affect the brain systems that are being studied. Medication use will be evaluated by our study physician on a case-by-case basis. For example, if the subject takes Benadryl but can safely refrain from use 24 hours prior to scanning sessions, they will not be excluded. * Significant history of past alcohol or substance abuse.