Cannabis use and suicidal thoughts and behavior: A multi-site, co-twin control study from adolescence through middle adulthood

Background: Observational studies have repeatedly linked cannabis use to increased suicidality, yet cannabis use rarely exists in isolation from other risk factors. We therefore tested whether these associations reflect a causal effect, reverse causation, or residual confounding.Methods: We analyzed twin cohorts based in Colorado and Minnesota (N = 6,420) followed from late adolescence into middle adulthood, with repeated assessments of cannabis use and suicidal thoughts/behavior (STB). Prospective and co-twin control analyses examined (a) cross-sectional and prospective links between cannabis use and STB, (b) whether these links persisted after adjusting for genetic and environmental confounds, and (c) whether middle-adulthood associations were moderated by legalized recreational cannabis.Findings: More frequent cannabis use was associated with elevated STB. After accounting for shared familial factors via co-twin control, this relationship remained significant in cross-sectional, but not prospective, analyses. In middle adulthood, associations between cannabis use and STB did not differ across states, regardless of recreational cannabis legalization status. Interpretation: Findings support a contemporaneous link between cannabis use and STB within twin pairs, potentially reflecting causal effects. However, the absence of a prospective within-pair association provides little support for the notion that cannabis use elevates future STB risk independent of familial liability. This pattern suggests causation may be limited to short-term effects, or that the cannabis-STB relationship may primarily reflect confounding by unmeasured time-varying factors (e.g., acute stressors).