It is well known in psychiatric clinics that a large proportion of schizophrenia patients smoke cigarettes and smoke heavily13. The dominant hypothesis to explain the comorbidity is self-medication12, i.e., that schizophrenia patients smoke to ameliorate impairments in cognitive function and suppress psychotic symptoms. Another hypothesis contends that schizophrenia and ND share some genetic liability, and the high prevalence rate of cigarette smoking in schizophrenia patients is a manifestation that is partially due to the common liability13. A third possibility is that smoking may be a risk factor for the development of schizophrenia, given that smoking initiation typically predates the onset of schizophrenia22. These three hypotheses are not mutually exclusive, and all three may contribute to the observed co-occurrence of schizophrenia and smoking.
