[OSF, supplement, previously] This study suggests that lifetime exposure to cannabis has few persistent effects on mental health and other psychosocial outcomes. The notable exceptions are cannabis use disorder, tobacco frequency, and illicit drug use, for which lifetime cannabis frequency causes small increases.
Background: Cannabis use is associated with outcomes like income, legal problems, and psychopathology. This finding rests largely on correlational research designs, which rely at best on statistical controls for confounding. Here, we control for unmeasured confounders using a longitudinal study of twins.
Method: In a sample of 4,078 American adult twins first assessed decades ago, we used cotwin control mixed effects models to evaluate the effect of lifetime average frequency of cannabis consumption measured on substance use, psychiatric, and psychosocial outcomes.
Results: On average, participants had a lifetime cannabis frequency of about 1–2× per month, across adolescence and adulthood.
As expected, in individual-level analyses, cannabis use was statistically-significantly associated with almost all outcomes in the expected directions.
However, when comparing each twin to their cotwin, which inherently controls for shared genes and environments, we observed within-pair differences consistent with possible causality in 3⁄22 assessed outcomes: cannabis use disorder symptoms (βW-Pooled = 0.15, SE = 0.02, p = 1.7 × 10−22), frequency of tobacco use (βW-Pooled = 0.06, SE = 0.01, p = 1.2 × 10−5), and illicit drug involvement (βW-Pooled = 0.06, SE = 0.02, p = 1.2 × 10−4).
[This is striking inasmuch as those 3 are unimpressive: you can’t have a marijuana disorder without having used marijuana, “illicit drug involvement” is also almost definitional, and “tobacco use” is another kind of smoking.]
Covariate specification curve analyses indicated that within-pair effects on tobacco and illicit drug use, but not cannabis use disorder, attenuated substantially when covarying for lifetime alcohol and tobacco use.
Conclusion: The cotwin control results suggest that more frequent cannabis use causes small increases in cannabis use disorder symptoms, ~1.3 symptoms when going from an once-a-year use to daily use. For other outcomes, our results are more consistent with familial confounding, at least in this community population of twins.