Cannabis use is a frequent problem among people with psychotic disorders that has been related to a worse prognosis. Understanding the reasons of cannabis use may help to develop comprehensive treatments and, as a result, improve the psychosis course. METHODS A systematic review of studies published in English and Spanish language from 1990 until March 2012 describing reasons for and subjective effects of cannabis use in patients with psychosis were reviewed. However, only those studies in which cannabis use was one of the three main substances studied were included. RESULTS Initially, 73 studies, of which 12 met the inclusion criteria, were identified. Most of the studies include heterogeneous patients, at different illness stages, and a few studies included a non-psychotic comparison group. The most common reasons for cannabis use were those related with social activities, mood disturbance, relaxation and getting high. CONCLUSIONS The most common reasons for cannabis use in patients with psychosis were related with social activities, mood disturbances, relaxation and getting high. However, a more homogeneous methodology need to be established, including different illness stages, to facilitate the interventions to reduce cannabis use in all phases of psychotic disorders development.