Study of Quetiapine Treatment for Cannabis Dependence

Inclusion Criteria: 1. Between the ages of 18-65 2. Meets DSM-IV criteria for current cannabis dependence 3. Seeking treatment for cannabis dependence 4. Reports using cannabis an average of five days per week over the past 28 days 5. Capable of giving informed consent and complying with study procedures Exclusion Criteria: 1. Lifetime history of DSM-IV diagnosis of schizophrenia, schizoaffective disorder, or bipolar disorder 2. Current DSM-IV criteria for any other psychiatric disorder that may, according to the investigator's judgment, require either pharmacological or non-pharmacological intervention over the course of the study 3. Receiving prescribed psychotropic medication 4. Known history of allergy, intolerance, or hypersensitivity to quetiapine 5. Pregnancy, lactation, or failure to use adequate contraceptive methods in female patients who are currently engaging in sexual activity with men 6. Unstable medical conditions, such as poorly controlled diabetes or hypertension, which might make participation hazardous 7. Current DSM-IV diagnosis of substance dependence other than cannabis or nicotine dependence 8. Are legally mandated to participate in a substance use disorder treatment program 9. Increased risk for suicide 10. Diabetes (whether controlled or not), hyperglycemia (fasting glucose \> 100 mg/dl), obesity (BMI \> 30) and elevated lipids (cholesterol \> 200 mg/dl; triglycerides \> 150 mg/dl).