Individuals with schizophrenia have a high risk of becoming addicted to drugs; between 13 to 42% of schizophrenics are addicted to marijuana. These individuals often have difficulties adhering to a substance abuse treatment program, and have an increased chance of marijuana relapse. Marijuana use by schizophrenics has also been associated with clinical exacerbations, noncompliance with antipsychotic medications, poor global functioning, and increased rehospitalization rates. While antipsychotic medications are often effective in controlling symptoms of schizophrenia, they are not always effective in preventing substance abuse. Clozapine, an atypical antipsychotic drug, is currently used to treat schizophrenia. Preliminary research has shown that clozapine is more successful at reducing drug relapse rates in individuals with schizophrenia, as compared to other antipsychotic medications, including olanzapine and risperidone. The purpose of this study is to compare the effectiveness of clozapine as compared to other oral antipsychotic treatment, including combinations of up to two antipsychotics, in reducing marijuana use in schizophrenic individuals.