Introduction: Mental disorder (MD) and substance use (SUD) are associated with poorer than average health and greater mortality. We analysed routine primary care (WLGP) and inpatient admission (PEDW) data to estimate incidence of co-occurring (CC) MD and SUD, and to estimate all-cause mortality and survival with CC, in children and young people in Wales, UK.

Methods: A retrospective population-based electronic cohort study using data from the Secure Anonymised Information Linkage (SAIL) Databank. Participants were 958,603 individuals aged 11 - 25 between 2008 and 2017. We estimated first ever incidence of CC, plotted Kaplanâ Meier survival curves and carried out Cox regression to estimate hazard ratios (HR) for risk of death by condition group (CC; MD or SUD only; NC).

Results: Higher incidence of CC in WLGP and PEDW was associated with male sex, older age and greater deprivation. Male to female IRRs (95% CI) were 1.18 in WLGP and 1.17 in PEDW, oldest to youngest IRRs were 24.80 in WLGP and 4.50 in PEDW and most to least deprived IRRs were 3.28 in WLGP and 2.59 in PEDW. Incidence in WLGP significantly decreased between 2008 and 2017; the greatest reduction occurred in the most deprived quintile. Incidence of hospital admissions remained stable. Risk of death was significantly higher for CC compared with NC.

Conclusion: Male sex, older age and greater deprivation were associated with higher CC incidence, although the gap between WIMD quintiles has decreased. CC was associated with greater risk of death. Mental health and substance misuse services should be provided in ways that remove barriers, and are acceptable and accessible to all, particularly those at greatest risk, or who are less likely to engage.