The association between cannabis and depression has not received as much attention as the links between cannabis use and psychosis. One of the reasons may be that the depressed are much less likely to come to the attention of treatment services than are those who are psychotic. Furthermore, some symptoms of cannabis dependence may mimic those of depression and so comorbid depression may go undiagnosed.
Rising rates of cannabis use (Donnelly and Hall, 1994; Hall et al., 1999; Degenhardt et al., 2000; Johns, 2001), depression (Andrews et al., 1998; Cicchetti and Toth, 1998) and suicide among young adults (Diekstra et al., 1995; Lynskey et al., 2000) have increased public concern about the role of substance abuse, including cannabis, in non-psychotic mental disorders. There has also been increasing advocacy for interventions to prevent and treat problematic cannabis use and depressed mood among young people. Given these parallel rises, recent speculation that the two may be linked is understandable.
Given the high prevalence of both cannabis use and depression there remains a question why any comorbid relationship has received little clinical attention? It may reflect a lack of association between the two. However, until recently, there was disagreement as to whether cannabis dependence (or problematic cannabis use) existed, with few treatments available. A lack of clinical attention may therefore have simply reflected a lack of services that might have detected an association. Lastly, due to its illegal status, cannabis use may remain unreported by clients presenting with depression.