The Effect of Smoking Cigarettes on the Progression of Psychotic Experiences to Psychotic Disorder: Evidence From a 6-Year Follow-Up Study

Background: Smoking cigarettes is highly prevalent in psychosis. However, the relationship between smoking and psychosis is still unclear. Aims: This paper aims to investigate the probable association in a 6-year follow up of community-based population. Methods: A general population sample were contacted at baseline ( n = 4,011) and at 6-year follow-up ( n = 2,185). Both at baseline and follow-up (newly developing/persistent), smoking cigarettes and psychotic experiences were assessed using the relevant sections of Composite International Diagnostic Interview 2.1. The associations between baseline smoking features and follow-up psychotic experiences were analysed using logistic regression models. To accurately assess the impact of smoking, participants were screened and controlled for alcohol and substance use. Interactions between baseline smoking features and existence of baseline psychotic experiences on the persistence of baseline psychotic experiences and its progression to psychotic disorder were analysed using interaction contrast ratios. Results: Follow-up psychotic experiences were 1.8 (OR = 1.8; CI [1.3, 2.6]) and follow-up psychotic disorders were 5.9 times (OR = 5.9; CI [2.9, 12.0]; p < .001) more common in individuals whose first smoking age were ⩽15 years old at T 1 and who were an active regular smoker. The risk of persistence (OR = 1.5, 95% CI [1.1, 2.2]) and progression to psychotic disorder (OR = 5.3, 95% CI [1.6, 17.8]) was higher in smokers compared to non-smokers. There was a significant additive interaction between baseline psychotic experiences and smoking on persistence of psychotic experiences at follow-up (ICR = 1.8; 95% CI [0.2, 3.6]). Conclusions: These findings suggest that early and regular cigarette smoking is associated with the persistence of psychotic experiences and increased risk of progression to psychotic disorder.