Smoking, symptoms, and quality of life in patients with psychosis, siblings, and healthy controls : a prospective, longitudinal cohort study

Background: The self-medication hypothesis postulates that the high prevalence of smoking in patients with psychosis can be explained by the ameliorating effect of smoking on symptoms. However, there are few large prospective studies testing this hypothesis. We aimed to examine the multi-cross-sectional and prospective associations of changes in smoking behaviour with symptoms and quality of life. Methods: In this prospective cohort study we recruited patients with a non-affective psychosis (n=1094), unaffected siblings (n=1047), and healthy controls (n=579). Patients aged between 16 and 50 years and diagnosed with a non-affective psychosis according to DSM-IV were recruited by clinicians from four university medical centres and 36 associated mental health-care institutions in the Netherlands and Belgium between Jan 13, 2004, and March 6, 2014. Smoking status and number of cigarettes per day were assessed at baseline, and at 3-year and 6-year follow-up using the Composite International Diagnostic Interview (CIDI). Symptom frequency was self-rated with the Community Assessment of Psychotic Experience (CAPE), and quality of life was assessed by the WHO Quality of Life (WHOQOL) schedule. Multiple linear mixed-effects regression analyses were done accounting for multiple confounders. Findings: At baseline, 729 (67%) of 1094 of patients smoked (mean 17·5 cigarettes per day, SD 8·8) compared with 401 (38%) of 1047 siblings and 145 (25%) of 579 healthy controls. Multi-cross-sectional results of linear mixed-effects analyses showed that smoking in patients and siblings was associated with more frequent positive symptoms (estimate 0·14, SE 0·02, p<0·0001 in patients; 0·03, 0·01, p=0·0019 in siblings), negative symptoms (0·15, 0·03, p<0·0001 in patients; 0·09, 0·02, p<0·0001 in siblings), and depressive symptoms (0·12, 0·03 p<0·0001 in patients; 0·08, 0·02 p<0·0001 in siblings) and lower quality of life (−0·59, 0·11, p