A causal relationship between controlled substances and firearm violence has been widely assumed in the United States, and federal law prohibits individuals who are "unlawful users of or addicted to any controlled substance" from purchasing or possessing firearms (68 FR 3750. 2003. Codified at 27 CFR §478.11). However, the law does a poor job of defining "unlawful users," resulting in recent calls for a revised, actionable definition. Such a definition should be informed by research evidence, but to date the epidemiologic research on the relationship between controlled substances and violence has not been comprehensively reviewed. The initial goal of this review was to summarize the best available evidence on the relationship between controlled substances and firearm violence, but only 1 study specific to firearm violence was identified. We therefore reviewed studies of this relationship using broader measures of interpersonal violence and suicide, all of which included but were not limited to firearm violence, and measures of illicit firearm carrying. Prospective longitudinal studies (n = 22) from 1990 to 2014 were identified by using searches of online databases and citation tracking. Information was extracted from each study by using a standardized protocol. Quality of evidence was independently assessed by 2 reviewers. Aggregate measures of controlled substance use were associated with increased interpersonal violence and suicide, but evidence regarding the relationship between specific substances and violence was mixed. Involvement in illegal drug sales was consistently associated with interpersonal violence. To effectively revise extant federal law and delineate appropriate prohibiting criteria, more research is needed to understand the relationship between controlled substances and firearm violence.
Keywords: controlled substances; drug trafficking; firearms; homicide; street drugs; violence.
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