Aims: We investigated the association between problematic use of alcohol and/or drugs (PUAD) and the incidence, urgency, and mode of discharge for a subsequent episode of self-harm (SH) or suicidal ideation (SI). Methods: This was a retrospective population-based cohort study of individuals admitted to hospital for an index episode of SH/SI (2010–2014) using linked data from hospital admissions and emergency department (ED) presentations. The outcome variables were (1) subsequent presentation to the ED for SH/SI, (2) triage category, and (3) mode of departure. Key predictors were PUAD. Results: In total, 23,007 individuals were admitted to hospital for an index SH/SI, of whom 8% had a subsequent presentation to an ED for SH/SI within a year. The odds of subsequent presentation was increased in those with problematic alcohol use (AOR 1.62, 95% CI 1.36, 1.92), drug use (AOR 1.28, 95% CI 1.07, 1.53), and mental health diagnoses (AOR 1.63, 95% CI 1.44, 1.85). Those with problematic alcohol use were more likely to be assigned to the most urgent triage categories (AOR 1.84, 95% CI 1.32, 2.56). Limitations: Defining SH and PUAD using administrative data is challenging, and the true prevalence is likely to be underestimated. Conclusion: The findings underscore the importance of drug health intervention as a key component of self-harm prevention.