Abstract
The association between cigarette smoking and schizophrenia is well established. However, up to 90% of people with schizophrenia begin smoking before the onset of their illness; thus, smoking could be an independent risk factor for schizophrenia. Prenatal exposure to maternal cigarette smoke is also associated with psychiatric problems in adolescence. Therefore, our aim was to undertake a systematic review and meta-analysis to explore the effect of smoking, and prenatal smoke exposure, on risk of schizophrenia. We systematically searched Medline, EMBASE, PsychInfo, Maternity and Infant Care, and Web of Science (from inception to February 2018) to identify comparative observational studies of the risk of schizophrenia in relation to smoking status. Measures of relative risk (RR) were pooled in a meta-analysis with 95% confidence intervals (CI), using random effects model. Twelve studies (9 cohort, 3 case-control) were included. Odds ratios (OR) and hazard ratios (HR) were pooled together to estimate pooled relative risks and estimates combined in a meta-analysis on an assumption of constant risk over time. Smokers had a significantly increased risk of schizophrenia compared with nonsmokers (RR = 1.99, 95% CI = 1.10% to 3.61%, I2 = 97%, 5 studies). Exposure to prenatal smoke increased the risk of schizophrenia by 29% (95% CI = 1.10% to 1.51%, I2 = 71%, 7 studies). Sensitivity analyses identified no significant differences between the results from studies reporting OR and hazard ratio. Our findings suggest smoking, and prenatal smoke exposure, may be an independent risk factor for schizophrenia. Care should be taken when inferring causation, given the observational nature of the studies. In this meta-analysis of 12 studies, smokers had a significantly increased risk of schizophrenia compared with nonsmokers. Exposure to prenatal tobacco smoke also increased the risk of schizophrenia by 29% compared with those with no exposure to prenatal tobacco smoke. Our findings suggest that smoking, and prenatal tobacco smoke exposure, may be independent risk factors for schizophrenia. These results may have important public health implications for decreasing the incidence of schizophrenia. The possibility of a causal link between smoking and schizophrenia warrants further investigation.
Highlights
The association between cigarette smoking and schizophrenia is well established
Since 90% of people with schizophrenia begin smoking before the onset of their illness 5-7 this suggests that smoking may increase the risk of schizophrenia; evidence of cognitive improvements in people with schizophrenia following smoking 8 9 supports the self-medication hypothesis
In view of this uncertainty we have carried out a systematic review to search the international literature for comparative observational studies of smoking and incident schizophrenia, including both personal active smoking and passive exposure to the foetus arising from maternal smoking in pregnancy, and used meta-analysis to estimate contemporary risks of schizophrenia
Summary
The association between cigarette smoking and schizophrenia is well established. up to 90% of people with schizophrenia begin smoking before the onset of their illness, smoking could be an independent risk factor for schizophrenia. It is possible that maternal smoking is associated with an increased risk of schizophrenia in offspring, since prenatal exposure to maternal cigarette smoke has been shown to be associated with psychiatric and behavioural problems in adolescence 10-12 and in adulthood 13-15. In view of this uncertainty we have carried out a systematic review to search the international literature for comparative observational studies of smoking and incident schizophrenia, including both personal active smoking and passive exposure to the foetus arising from maternal smoking in pregnancy, and used meta-analysis to estimate contemporary risks of schizophrenia
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