However, they also acknowledged that schizophrenia itself might increase the risk of individuals starting to use cannabis and other substances – a theory supported by the wide range of substances associated with conversion to schizophrenia.
“In our study, the first registered date of substance use disorder came before the first registered date of schizophrenia, but delays in diagnoses may have caused schizophrenia or its preclinical symptoms to predate substance use.”
The association between alcohol use disorder and conversion to schizophrenia was seen only in sensitivity analyses that ignored the birth year of the cohort, which the authors suggested may have been the result of increased power in this analysis.
Among the limitations cited by the authors was their inability to validate the schizotypal disorder diagnoses. Also, they wrote, the participants’ diagnoses were based on ICD-8 and ICD-10 criteria, and it was unclear whether the study results are generalizable to DSM-5 diagnoses.
The Lundbeck Foundation supported the study. No conflicts of interest were declared.
SOURCE: Hjorthøj C et al. JAMA Psychiatry. 2018 Apr 25. doi: 10.1001/jamapsychiatry.2018.0568.