As it turns out, one of the subgroup analyses showed that the association between schizophrenia and breast cancer was significant only in studies that excluded women who were diagnosed with breast cancer before they were diagnosed with schizophrenia (standardized incidence ratio, 1.34; 95% CI, 1.20-1.51; P less than .001).
The same was seen in studies where there were more than 100 cases of breast cancer (SIR, 1.31; 95% CI, 1.18-1.46; P less than .001), while the association was not significant in studies with fewer than 100 cases.
The authors said their findings contradict a hypothesis that schizophrenia might be protective against cancer.
“These results, together with our recent meta-analysis results showing no association with lung cancer risk but a reduced hepatic cancer risk in schizophrenia, indicated that the association between schizophrenia and cancer risk may be complicated and depend on the cancer site,” wrote Dr. Zhuo and Dr. Triplett.