Comparison With Existing Literature
Our finding of an elevated risk of developing schizophrenia in cannabis users not only supports similar findings from previous longitudinal studies (1) but also contributes
to this literature by showing that an even greater risk of subsequent schizophrenia can be observed in a group of (likely) heavy cannabis users having a severity that warranted a hospital diagnosis. To our knowledge, no data are available from large-scale, sufficiently powered longitudinal studies specifically investigating the possible
influence of the other drugs of abuse we examined here in relation to schizophrenia, and our findings therefore require independent replication (27). In this regard, one of
the original Swedish cohort investigations (28), although focused on cannabis, reported a significant univariate association between risk of schizophrenia and use of stimulants. The authors noted, however, that this effect did not appear in the multivariate analyses, and the study design was not adequately powered to address this question.