Cannabis involvement and mass shooting events in the United States from 1900 to 2019.

Objectives: Mass murder is associated with a lifetime history of substance use. We aimed to examine cannabis involvement among those who committed mass shootings in the United States from 1900 to 2019.

Methods: We identified mass shooting events in the United States from 1900 to 2019 using publicly available English-language media reports and court/police records. People who perpetrated mass murders using methods other than firearms (eg, knives, automobiles) were used as a comparison group. Events were dichotomised into either prior to 1996 or from 1996 onward (first legalisation for medical use by California). Post-1960 data were used for additional analyses of a more modern era.

Results: The proportion of those who committed mass shootings who had used, possessed, and/or distributed cannabis was significantly higher for events that occurred from 1996 onward, compared with prior to 1996 (11.2% vs 4.9%, p = 0.002). The proportion of those committed mass murders by other methods who had used, possessed, and/or distributed cannabis did not significantly differ for events that occurred from 1996 onward, compared with prior to 1996 (4.8% vs 5.7%, p = 0.76). When 58 mass shooting events and 31 mass murder events by other methods perpetrated before 1960 were excluded, results were similar when 1996 was used as a cutoff for the respective events (p = 0.02 and p = 0.40). Among those who committed mass shootings, those with cannabis involvement (n = 74) were younger than those without (n = 754) [28.7 vs 33.5 years, p < 0.001] and were of younger age group than older age group (11.9% vs 5.8%, p = 0.002).

Conclusion: Cannabis use may be harmful in subgroups of individuals (eg, those who committed mass shootings) who are vulnerable to cannabis use. This should be considered by policymakers, individuals with commercial interests, the public, and mental health and medical professionals when they debate related public health issues.