Are early-onset cannabis smokers at an increased risk of depression spells?

Background: A recent research focus is a set of hypothesized adult-onset mental health disturbances possibly due to early-onset cannabis use (EOCU, onset <18 years). We seek to estimate the suspected EOCU-associated excess odds of experiencing an incident depression spell during adulthood, with comparisons to never cannabis smokers and those with delayed cannabis onset (i.e., not starting to smoke cannabis until adulthood).

Methods: The National Surveys on Drug Use and Health (NSDUH) assess non-institutionalized community-dwelling residents of the United States after probability sampling each year. In aggregate, the NSDUH analytical sample included 173,775 adult participants from survey years 2005-2009 (74-76% of designated respondents). Standardized computer-assisted interviews collected information on background determinants, age of first cannabis use, and depression spell onset. Logistic regression was used to estimate EOCU-depression spell associations in the form of odds ratios, with statistical adjustment for sex, age, race/ethnicity, years of cannabis involvement, tobacco cigarette onset, and alcohol onset.

Results: About 1 in 10 experienced a depression spell during adulthood, and both early-onset and adult-onset cannabis smokers had a modest excess odds of a depression spell compared to never cannabis smokers, even with covariate adjustment (OR=1.7 and 1.8, respectively; both p<0.001). Estimates for early- and adult-onset cannabis smokers did not statistically differ from one another.

Limitations: Shared diathesis that might influence both EOCU and adult-onset depression spell is controlled no more than partially, as will be true until essentially all known early-life shared vulnerabilities are illuminated.

Conclusion: Cannabis smoking initiated at any age signals a modest increased risk of a spell of depression in adulthood, even when adjusted for suspected confounding variables studied here. Delaying cannabis onset until adulthood does not appear to diminish the cannabis-associated risk.