Context:
Previous studies of the association between schizophrenia and cancer have produced conflicting results, probably because of the failure to control for confounding factors.
Objective:
To test if the possible association between schizophrenia and cancer is genetic by investigating the incidence of cancer in patients with schizophrenia and their relatives.
Design:
Retrospective cohort study with follow-up between 1965 and 2008. Estimated smoking rates were used to adjust the incidence rates of smoking-related cancers.
Participants:
The entire Swedish population.
Main outcome measures:
Risk of overall cancer and 34 site-/type-specific cancers.
Results:
A total of 59 233 patients in Sweden with schizophrenia were identified, of whom 6137 developed cancer during the study period, giving a decreased standardized incidence ratio (SIR) of 0.79 (95% CI 0.77–0.81). The decrease was more pronounced (SIR 0.40, 95% CI 0.38–0.43) before the first diagnosis of schizophrenia. The overall risk was significantly reduced among their unaffected parents (SIR 0.96, 95% CI 0.94–0.98) and siblings (SIR 0.92, 95% CI 0.89–0.96). Sex-stratified analyses indicated different incidence rates between males and females, with female patients having higher cancer risks than the general population.
Conclusions:
The significantly decreased incidences of cancers in patients diagnosed with schizophrenia and their unaffected relatives suggest that familiar/genetic factors contributing to schizophrenia may protect against the development of cancer, especially for those cancer sites observed in both settings. The increased risk of breast, cervical, and endometrial cancers after the first diagnosis of schizophrenia could be attributed to nongenetic factors such as antipsychotics administration, which may justify preventive medical screening.