Income Inequality as a Social Determinant of Opioid Use Disorder

Background: In 2015, there are 251.3 opioid use related inpatient hospitalizations per 100,000 population in the US. Over 33,000 Americans lost their lives due to overdoses from opioids in the same year (National Academies of Sciences, 2017). We explore the association between income inequality and opioid use disorder as a test of the income inequality hypothesis (IIH) in the social epidemiology literature.

Data: We obtained the hospital discharge data from the Agency for Healthcare Research and Quality’s National Inpatient Sample 2011 collected among participating hospitals and states in the US. We selected a sample of adult patients aged 18 or over for analysis. Opioid use disorder is defined as having at least one of the following conditions, opioid abuse, adverse effects of opioids, opioid dependence and unspecified use, and opioid poisoning. The individual-level data were linked with income inequality measurements calculated at the state-level using the American Community Survey five year estimates.

Statistical Method: Multilevel linear regression was used. We choose to use linear probability model due to the simplicity in interpretation of its results and the computation burden associated with the large number of observations. Confounding factors include age, sex, rurality, and quartiles of zip-code mean income.

Result: A ten percent point increase in Gini coefficient (measure of income inequality) is associated with 0.9% increase in the probability of opioid use disorder in 2011.

Conclusion/Discussion: We find evidence of income inequality as a social determinants of opioid use disorder. Potential causal mechanisms of IIH include stress induced due to social comparison, reduced social capital, and differences in social polices associated with higher income inequality across states. Further research will explore the association between zip-code level income inequality and opioid use disorder in multiple years.