This chapter reviews studies that have examined the effects of cannabis use on the course of schizophrenia. It suggests that intensity of abuse is correlated with an increase of psychotic relapses, suggesting delta-9-tetrahydrocannabinol (THC) interferes with dopaminergic neurotransmission in the medial forebrain of patients. Future studies with brain imaging techniques applied to heavy and non-abusing schizophrenic patients with standard antipsychotic medication may reveal these differences in dopamine receptor blockade or in other neurotransmission systems (e.g. glutamate). Cannabis abuse may also influence antipsychotic drug metabolism, lowering plasma levels of active metabolites. Thus, theoretically cannabis abusers with schizophrenia could be relatively undertreated. It is also possible that young persons who use cannabis regularly are more vulnerable to, or have less effective coping mechanisms for dealing with, stressful life events. Further studies are needed to elucidate the relationship between cannabis abuse and psychotic symptoms in schizophrenia.