Disparities in screening participation and stage at diagnosis for breast and bowel cancers among individuals with pre-existing mental health disorders: A population-based study

People with pre-existing mental health disorders (PMHD) often have lower cancer screening uptake and later-stage diagnoses. It remains unclear whether these disparities persist within highly accessible universal healthcare systems without strict gatekeeping. We simultaneously investigated the associations between PMHD and both screening participation and stage at diagnosis of breast and bowel cancers within the Taiwan health service. Using nationwide National Health Insurance and Cancer Registry data covering over 99% of the population, we compared outcomes between individuals with and without PMHD. Propensity score matching was applied for the screening cohort. Adjusted odds ratios were estimated using logistic regression, with subgroup analyses for severe PMHD. Overall, PMHD was associated with higher screening participation (adjusted odds ratios [aOR]: breast = 1.04, bowel = 1.12) and reduced odds of metastatic disease at diagnosis (breast = 0.72, bowel = 0.93), although the magnitude of these associations was modest, particularly for bowel cancer. Conversely, individuals with severe PMHD had significantly lower odds of screening (breast = 0.81, bowel = 0.88) despite high outpatient use. Furthermore, while the severe group retained reduced odds of metastatic breast cancer at diagnosis (aOR = 0.79), they showed no staging advantage for bowel cancer. While Taiwan's highly accessible universal healthcare system may contribute to a surveillance effect benefiting the general PMHD population, significant disparities persist for individuals with severe illnesses. Targeted interventions, procedural simplifications for self-administered tests, and multidisciplinary person-centred care models, may be needed to ensure health equity.