Statement of Purpose To determine the risk of emergency department (ED) or hospital readmission with new Drug Use Disorder (DUD) among patients who survive an index firearm injury, with and without mental illness at baseline, and if the risk varies based on treatment pathway.

Methods We conducted a retrospective cohort study of drug-use naïve patients surviving index firearm injury treated in emergency departments (ED) in California from 2006–2010. We compared patients with and without mental illness at baseline and treatment pathway (ED alone and ED+hospitalization). Of the 18,941 treated in ED alone and 10,370 with ED+hospitalization, 1,875 (9.9%) and 2,568 (24.8%) had pre-existing mental illness at index injury.

Results Patients with mental illness had a greater risk of readmission with new DUD at one year [5.6% vs 2.1%; adjusted Hazard Ratio (aHR) 2.04 (95% CI 1.60, 2.60)] than those without. In patients not hospitalized, 7.7% of those with mental illness were readmitted with incident DUD versus 2.2% for those without (aHR=2.99, 95% CI 2.18, 4.09). No such risk was demonstrated in hospitalized patients. The risk of readmission with incident opioid use at 1-year among patients with and without mental illness was 1.5% versus 0.6%, [aHR=1.81 (95% CI 1.13, 2.90)].

Conclusion Patients exposed to firearm injury with mental illness have an increased risk of 1-year readmission with new DUD when treated in ED without hospitalization. This vulnerable population may benefit from an additional evaluation, treatment, and possible hospitalization, to reduce the risk of future DUD.

Significance Our study reveals the impact of identifying patients with pre-existing mental illness at the time of firearm injury, considering mental illness in treatment decisions, and addressing mental illness as a risk factor for post-injury complications, regardless of their injury severity, to avoid the development of drug use and dependence that may lead to further adverse outcomes.