Diagnostic sequence of chronic pain and severe mental illness: Relationship with mental health and hospitalization outcomes

Background Chronic pain (CP) and severe mental illness (SMI) frequently co-occur, exacerbating clinical outcomes and healthcare burden. The influence of diagnostic sequence, that is, whether CP precedes SMI or vice versa, on mental health and hospitalization remains underexplored. This study investigates how the temporal order of CP and SMI diagnoses impacts illness burden, mental health comorbidities, and hospital admissions. Methods Using linked electronic health records from primary care, mental health, and acute care in South London, we identified 1112 patients with comorbid CP and SMI (e.g. schizophrenia). Patients were grouped by diagnostic order: CP before SMI (CP-SMI, n = 413) versus SMI before CP (SMI-CP, n = 699). Outcomes included illness burden, mental health comorbidities, and hospital admissions, which were analysed via multivariable regression, adjusting for confounders. Results Compared to the CP-SMI group, the SMI-CP group exhibited significantly lower odds of self-injury (OR = 0.55, 95% CI: 0.31–0.98), substance misuse (OR = 0.55, 95% CI: 0.34–0.86), depressive mood (OR = 0.63, 95% CI: 0.44–0.89), and depression diagnosis (OR = 0.58, 95% CI: 0.43–0.78). Among the SMI-CP group, older age reduced the odds of self-injury, substance misuse, and depressive mood but increased the risks of cognitive and physical problems. No significant differences were observed in psychiatric or general hospital admissions or physical comorbidities, though ethnic disparities persisted in psychiatric admissions. Conclusion Patients diagnosed with CP before SMI experience worse mental health outcomes. This is likely due to delayed psychiatric intervention, therefore routine psychiatric screening aiming to improve outcomes and reduce healthcare burden at CP diagnosis should be evaluated.