The issue of self medication was addressed by Henquet and colleagues , 6 using data from the German prospective early developmental stages of psychopathology study . 12 13 The authors investigated the association between cannabis use at baseline and subsequent development of psychotic symptoms at four year follow - up and reported that after adjustment for pre - existing psychotic symptoms , cannabis use at baseline still remained significantly associated with psychotic symptoms at follow - up . There was no evidence of an effect of self medication as pre - existing psychotic symptoms did not significantly predict later cannabis use . 6 Ferdinand and co - workers investigated the role of pre - existing self reported psychotic symptoms and showed a bi - directional association between cannabis and psychotic symptoms over a 14 year follow - up study in the general population . 11 They showed that cannabis use predicted later psychotic symptoms in individuals with no evidence of psychotic symptoms before starting to use cannabis and that the reverse was also true , in that psychotic symptoms predicted cannabis use in those who had not used cannabis before the onset of those symptoms . 11 A prospective population based cohort study also found evidence for a self medication effect . 14 Individuals with self reported hallucinations at the age of 14 had a higher risk of using cannabis on a daily basis at the age of 21 . In a sibling pair analysis , however , this study also suggested an independent effect of cannabis use on self reported delusional ideation later in life . 14 Thus , although the cannabis - psychosis link has been investigated in many studies , results on the temporal association between cannabis use and psychotic symptoms remain conflicting . Longitudinal cohort studies with multiple repeated interview based measures of cannabis use and psychotic symptoms are needed to clarify this issue . The EDSP study , 12 13 which completed its recent 10 year follow - up representing the fourth assessment ( assessments at baseline , T1 , T2 , and T3 , see also fig 1 ) , is uniquely suitable for the investigation of the temporal association between cannabis and psychosis .