Many individuals with schizophrenia abuse cannabis at the onset of their illness, portending a poorer course of illness and poorer treatment response. Preliminary evidence suggests that clozapine may uniquely reduce substance use in patients with schizophrenia. The purpose of this study is to establish an effective methodology for studying early treatment with clozapine in patients with co-occurring schizophrenia and cannabis use disorder, while generating pilot data comparing clozapine vs. risperidone on substance use, psychiatric symptoms, side effects, and treatment discontinuation.