Investigating the association between cigarette smoking and schizophrenia in a cohort study

Objective: Many case-control and cross-sectional studies have observed an association with cigarette smoking after the onset of schizophrenia, and there is evidence to suggest that smoking may improve symptoms in people with this disorder. Here, the authors investigated whether cigarette smoking alters the risk of subsequently developing schizophrenia. No longitudinal studies have previously examined this relationship. Method: A cohort of 50,087 Swedish conscripts (98% were ages 18-20) was followed up by record linkage to the National Register of Inpatient Care from 1970 to 1996 to determine hospital admission for schizophrenia. Cox regression was used to obtain hazard ratios and 95% confidence intervals (CIs) for schizophrenia, according to smoking status. Results: Smoking cigarettes at ages 18-20 was associated with a lower rate of developing schizophrenia after adjustment for confounders. There was a linear relationship between the number of cigarettes smoked and a lower risk of schizophrenia (adjusted hazard ratio for linear trend across smoking categories, 0.8 [95% CI=0.7-0.9]), with an adjusted hazard ratio for heavy smokers of 0.5 (95% CI=0.3-0.9) compared to that of nonsmokers. This association persisted when analysis was restricted to subjects diagnosed after the first 5 years following conscription to reduce possible prodromal effects of schizophrenia on smoking. Conclusions: Cigarette smoking may be an independent protective factor for developing schizophrenia. These results are consistent with animal models showing both neuroprotective effects of nicotine and differential release of prefrontal dopamine in response to nicotine. The harmful effects of cigarette smoking vastly outweigh any possible benefits, but nevertheless, further investigation may lead to important insights regarding the etiology of schizophrenia at a molecular level.