Improving Substance Use and Clinical Outcomes in Heavy Cannabis Users With Quetiapine

Inclusion Criteria: * DSM-IV-defined diagnosis of cannabis dependence (304.30) assessed with the Structured Clinical Interview for Axis I DSM-IV Disorders (SCID-I/P) (First 1998)) * one or more attenuated positive symptoms with a score 3 ('moderate'), 4 ('moderate severe'), or 5 ('severe but not psychotic') during the past year assessed with the Structured Interview for Prodromal Syndromes (SIPS) and the Scale of Prodromal Symptoms (SOPS) (McGlashan et al, 2001) * lifetime treatment with antipsychotic medication less than 2 weeks * cannabis use for more than one year * cannabis use three or more days per week on average for the past 3 months * aged 18 to 65 * competent and willing to sign informed consent * for women, a negative urine pregnancy test and agreement to use a medically accepted method of birth control during the study. Exclusion Criteria: * DSM-IV criteria for schizophrenia, schizophreniform disorder, schizoaffective disorder, a psychotic disorder due to a general medical condition, psychosis NOS, delusional disorder, brief psychotic disorder, shared psychotic disorder, or a mood disorder (major depression or bipolar) with psychotic features assessed with the Structured Clinical Interview for Axis I DSM-IV Disorders (SCID-I/P) (First 1998) * DSM-IV diagnosis of any psychoactive substance dependence other than cannabis or nicotine * being in an environment with no access to cannabis (e.g., hospitalization, residential treatment, jail, ..) for more than one week during the past three months preceding study entry * serious neurological or endocrine disorder or any medical condition or treatment known to affect the brain * use of medications that have an effect on monoamines (e.g., antidepressants * severe medical or physical illnesses * criteria of the National Cholesterol Education Program (NCEP) for a metabolic syndrome (Expert panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults, 2001) * medical condition that requires treatment with a medication that has psychotropic effects * significant risk of suicidal or homicidal behavior * cognitive or language limitations, or any other factor that would preclude subjects providing informed consent or participating in study procedures * treatment with medications for addiction * treatment with medication having a risk of addiction (e.g., benzodiazepines, barbiturates) * history of treatment resistance to quetiapine ER * medical contraindications to quetiapine ER * hypersensitivity to quetiapine ER or any of its component * for women, pregnant, breastfeeding, or intention to become pregnant during the study timeframe