Schizophrenia in users and nonusers of cannabis. A longitudinal study in Stockholm County

In a previous longitudinal study of Swedish conscripts we have shown strong association between level of cannabis consumption at conscription and development of schizophrenia during 15 years of follow-up. In this study we further analysed data from a subsample of the national cohort. Case records for all conscripts residing in Stockholm County who reported consuming cannabis on more than 10 occasions and who subsequently developed schizophrenia (n = 8) were compared with case records of a sample of conscripts who also developed schizophrenia but reported no cannabis consumption at conscription (n = 13). The relative risk of schizophrenia among cannabis users in Stockholm County was 4.1 (95% confidence interval 1.8-9.3) compared with nonusers. No evidence was found of a significant role for any other narcotic drug in the emergence of schizophrenia among cannabis abusers. Further, there was no evidence of mental disorder prior to cannabis abuse, even if the role of personality traits could not be fully assessed. A different pattern of mental deterioration was found among cannabis users, with a more abrupt onset of schizophrenic symptoms than nonusers. There was no major difference between users and nonusers in heredity for schizophrenia or other mental disorder. Negative social background factors were more common among cannabis abusers. Although the number of cases in this study was small, the findings support the hypothesis that cannabis does play an aetiological role in schizophrenia.