Associations of cannabis and tobacco use with depression at age 18: findings from the Avon Longitudinal Study of Parents and Children
METHODS: Data from ALSPAC, a prospective longitudinal cohort study based in Bristol, UK, were used. Cannabis and cigarette use were measured at age 16 via questionnaire. Depression was assessed at age 18. Logistic regression was used to investigate associations between cannabis or cigarettes and depression, before and after adjustment for pre-birth, childhood and adolescent confounders.
RESULTS: Cannabis use and depression were associated in the unadjusted relationship (OR 1.52, 95% CI 1.26-1.84). Adjustment for pre-birth and childhood confounders did not attenuate the relationship greatly. Nor did further adjustment for cigarette use, or other illicit drug use. Adjustment for alcohol slightly attenuated the relationship (OR 1.36, 95% CI 1.08-1.72). Cigarette use and depression were also associated (OR 1.36, 95% CI 1.14-1.63). Adjustment for pre-birth and childhood confounders attenuated the results only marginally, but further adjustment for cannabis use fully attenuated this association (OR 0.96, 95% CI 0.74, 1.25). Adjustment for alcohol or other illicit drugs also attenuated the findings, but not to the same degree as adjusting for cannabis did, though CIs still crossed the null.
CONCLUSIONS: The association between cannabis and depression was robust to all confounding by measured variables, although alcohol led to some attenuation. Associations of tobacco with depression showed a different pattern, where adjustment for cannabis use, illicit drug use or alcohol attenuated the association. Tobacco and cannabis use are collinear, which makes teasing out independent effects hard. Complementary methods are required to robustly examine independent effects of cannabis and tobacco on depression.