The incidence and relative risk factors for developing cancer among patients with schizophrenia: a nine-year follow-up study

Objective: To estimate the incidence and relative risk of developing cancer as well as the mortality rate after cancer diagnosis for patients with schizophrenia compared with the general population.

Methods: Our population for this study was identified before the end of 1999. The study included 59,257 patients with schizophrenia and 178,156 age- and gender-matched individuals without schizophrenia as controls, who were selected from the 23,981,020 subjects in the National Health Insurance Research Database (NHIRD), which consists of 96% of the entire Taiwanese population. From the 2000 to 2008 NHIRD, we calculated the cancer incidence and survival time after cancer diagnosis in each of the two groups. Based on the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM), the cancers were divided into nine groups.

Results: During the nine-year follow-up period, 1145 (1.93%) of the patients with schizophrenia and 5294 (2.97%) of the control group developed cancer. The patients with schizophrenia had a significantly lower cancer incidence than those in the control group in both the male (OR=0.50, 95% CI, 0.46-0.55) and female (OR=0.81, 95% CI, 0.74-0.88) populations. Patients with schizophrenia were less likely to develop cancer than individuals in the control group for every cancer type except breast and cervical/uterine cancer. After adjustment using the Cox regression model, patients with schizophrenia had an overall decreased cancer risk (adjusted hazard ratio 0.71, 95% CI, 0.66-0.76) compared to the control population. For all cancer patients, the mortality adjusted hazard ratio for patients with schizophrenia versus the control group was 1.36 (95% CI, 1.24-1.50) after adjusting for other variables.

Conclusions: Although the likelihood of developing cancer among patients with schizophrenia (0.64) was less than that of the non-schizophrenia group, the mortality rate among patients with schizophrenia was higher than that of the control group.