Cannabis is the most prevalent illicit substance used among schizophrenia patients. The effects of cannabis are mediated through the endocannabinoid system, which is a major regulator of neurotransmission and may be disturbed in schizophrenia. Though cognitive impairment in schizophrenia is well established, the effects of cannabis on cognition in schizophrenia patients are still unclear. This paper reviews 19 studies that examine the cognitive effects of cannabis on schizophrenia by comparing cognitive functioning of cannabis-using and non-using schizophrenia patients across a vast range of domains (memory, attention and processing speed, executive functions, visuospatial, psychomotor and language). Of the studies included in the review, 11 reported better cognitive functions among cannabis-using schizophrenia patients compared to non-users, 5 found minimal or no difference between the groups and 3 found poorer cognitive functions among cannabis-using schizophrenia patients compared to non-users. The inconsistencies in the studies reviewed may stem from significant methodological variance between the studies regarding patient selection, adequate controls, cognitive measures used, measures of cannabis use, additional drugs used, and clinical aspects of schizophrenia. These methodological issues are discussed, as well as possible explanations for the results presented and suggestions for future research in this field.