Cigarette smoking as a risk factor for schizophrenia or all non-affective psychoses

Background Smoking tobacco is regarded as an epiphenomenon in patients with schizophrenia when it may be causal. We aimed to examine whether smoking status is related to the onset of schizophrenia or the broader diagnosis of non-affective psychosis, including schizophrenia. Methods We used data from The Health Improvement Network primary care database to identify people aged 15-24 between 1 January 2004 and 31 December 2009. We followed them until the earliest of: first diagnosis of schizophrenia (or psychosis), patient left the practice, practice left THIN, patient died or 31 December 2014. Results In men, incidence rates for schizophrenia per 100 000 person years at risk were higher in smoking initiators (non-smoker who became a smoker during the study) than in non-smokers (adjusted IRR 1.94; 95% CI 1.29-2.91) and higher still in smokers (adjusted IRR 3.32; 95% CI 2.67-4.14). Among women, the incidence rate of schizophrenia was higher in smokers than in non-smokers (adjusted IRR 1.50; 95% CI 1.06-2.12), but no higher in smoking initiators than non-smokers. For non-affective psychosis, the pattern was similar for men but more evident in women where psychosis incidence rates were higher in smoking initiators (adjusted IRR 1.90; 95% CI 1.40-2.56) and in smokers (adjusted IRR 2.13; 95% CI 1.76-2.57) than in non-smokers. Conclusions We found an important and strong association between smoking and incidence of schizophrenia. Smoking may increase risk through as yet unknown pathways or smoking may share genetic risk with schizophrenia and non-affective psychoses.