A reanalysis and extension of the same Swedish conscript cohort reconfirmed that heavy cannabis users by the age of 18 years were 6.7 times more likely than non-users to be hospitalized for schizophrenia in the following 27 years [239]. This study addressed the confounding effects of concomitant use of other drugs of abuse, pre-morbid personality traits, and cannabis use as a form of self medication of schizophrenia. The adjusted odds ratio for cannabis use and schizophrenia remained significant (1.2), despite adjusting for a number of confounds including low IQ, urbanicity, cigarette smoking, poor social integration, occupational function, and stimulant use. Further, even after excluding subjects who developed schizophrenia within 5 years of conscription in an effort to control for the possibility that cannabis use was a consequence of prodromal manifestations of psychosis, the finding of an increased risk of schizophrenia conferred by cannabis use persisted. The authors concluded that cannabis use was associated in a causal way with an increased risk of developing schizophrenia.
