Social Isolation May Cause Cannabis Use Disorder, With Depression Explaining About 22% of the Effect

Mendelian randomization analysis showed social isolation was causally associated with a 4.29-fold increased risk of cannabis use disorder (OR=4.29, 95% CI: 1.35-13.64). Social isolation also significantly increased depression risk (OR=3.70), and depression in turn increased CUD risk (OR=1.27). Depression mediated 21.8% of the social isolation to CUD pathway.

Two-sample Mendelian randomization using GWAS summary statistics for social isolation, cannabis use disorder, anxiety disorders, and depression from public repositories. Multiple MR methods (IVW, MR-Egger, weighted median, maximum likelihood) with extensive sensitivity analyses.

While observational studies have linked loneliness to substance use, this genetic approach provides stronger evidence for a causal relationship. The finding that depression mediates over one-fifth of the effect suggests treating depression in socially isolated individuals could help prevent cannabis use disorder.

This study connects three major public health concerns: social isolation, depression, and cannabis use disorder. The causal chain it identifies (isolation leads to depression, which leads to CUD) suggests that addressing loneliness and depression could be upstream prevention strategies for substance use.

European ancestry samples only. Mendelian randomization assumes no horizontal pleiotropy (tested but not guaranteed). Summary-level data cannot assess exposure-mediator interactions. Cannabis use disorder definitions may vary across GWAS sources. Effect sizes have wide confidence intervals.