Although substance use disorders have long been considered a public health problem, new developments in the supply, trade, and misuse or abuse of substances have prompted researchers, practitioners, and policymakers to cast a more critical eye towards their root causes in order to develop more effective drug policy. While historical approaches to substance use disorders have primarily focused on abstinence and law enforcement approaches to stem the illicit production and supply of controlled substances, more recent research and practice has led to policy based on the harm reduction paradigm which seeks to mitigate the harm from substance abuse and misuse rather than eliminating use altogether as a goal. Yet, there has been a growing recognition that harm reduction alone is insufficient to address the rapidly evolving problem of substance use disorders in society, particularly considering new literature from medical sociology which has underscored the importance of identifying and addressing disparities in health outcomes based upon social determinants. As such, this dissertation seeks to contribute to our understanding of the scale of substance use disorders among adults in the United States by analysing social disparities in the epidemiology and, using the opioid epidemic as a case study, treatment of substance use disorders with the intention of highlighting potential areas for intervention and policy development. Drawing upon administrative datasets funded and maintained by the Substance Abuse and Mental Health Services Administration, specifically the National Survey for Drug Use and Health, the National Survey of Substance Treatment Services, and the Treatment Episode Data Set, I examine disparities in the prevalence of substance use disorders in the United States and expressed unmet need for mental health treatment. Using the opioid epidemic as a case study for the treatment of substance use disorders, I examine disparities in service provision among substance abuse treatment facilities as well as disparities in receipt of medication-assisted therapy among admitted patients. I also present an analysis of substance use disorders in England and Wales drawn from the Crime Survey for England and Wales for context. The findings of these analyses are presented with an overarching aim of informing sound policy and practice with respect to substance-related use disorders.