Does smoking carry increased schizophrenia risk?

Smoking may contribute to a higher incidence of schizophrenia, as suggested in a recent study.

Researchers used data from The Health Improvement Network primary care database and included 907,586 individuals. They recorded smoking status of participants within the first 12 months after study entry and followed them for a median of 6 years for the incidence of schizophrenia or nonaffective psychotic disorder, with the observation period commencing at 12 months after entry.

A total of 570,457 individuals (62.9 percent) were classified as nonsmokers over the 12-month observation period, while 266,396 (29.4 percent) had started smoking prior (smokers), and 70,733 (7.8 percent) started smoking during follow-up (smoking initiators). The mean age at first record of smoking was 21.1 years in males and 20.0 years in females.

In men, schizophrenia was diagnosed more frequently among smoking initiators (nonsmoker who became a smoker during the study) than nonsmokers (adjusted incidence rate ratio [IRR], 1.94, 95 percent CI, 1.29–2.91), and even higher in smokers (adjusted IRR, 3.32, 95 percent CI, 2.67–4.14).

In women, schizophrenia incidence was higher in smokers than in nonsmokers (adjusted IRR, 1.50, 95 percent CI, 1.06–2.12), but was similar in smoking initiators and nonsmokers.

For nonaffective psychosis, the pattern was similar for men but more evident in women where psychosis incidence was much higher in smoking initiators (adjusted IRR, 1.90, 95 percent CI, 1.40–2.56) and in smokers (adjusted IRR, 2.13, 95 percent CI, 1.76–2.57) than in nonsmokers.

The present data suggest that smoking may bear an increased risk of schizophrenia through as yet unknown pathways or shared genetic risk with the mental disorder and other nonaffective psychoses, the researchers said.