Previous research has reported both a moderate degree of comorbidity
between cannabis dependence and major depressive disorder (MDD) and that early-onset
cannabis use is associated with increased risks for MDD.
To examine whether associations between both lifetime cannabis dependence
and early cannabis use and measures of MDD, suicidal ideation, and suicide
attempt persist after controlling for genetic and/or shared environmental
Cross-sectional survey of twin pairs discordant for lifetime cannabis
dependence and those discordant for early cannabis use.
General population sample of twins (median age, 30 years).
Two hundred seventy-seven same-sex twin pairs discordant for cannabis
dependence and 311 pairs discordant for early-onset cannabis use (before age
Main Outcome Measures
Self-report measures of DSM-IV–defined
lifetime MDD, suicidal ideation, and suicide attempt.
Individuals who were cannabis dependent had odds of suicidal ideation
and suicide attempt that were 2.5 to 2.9 times higher than those of their
non–cannabis-dependent co-twin. Additionally, cannabis dependence was
associated with elevated risks of MDD in dizygotic but not in monozygotic
twins. Those who initiated cannabis use before age 17 years had elevated rates
of subsequent suicide attempt (odds ratio, 3.5 [95% confidence interval, 1.4-8.6])
but not of MDD or suicidal ideation. Early MDD and suicidal ideation were
significantly associated with subsequent risks of cannabis dependence in discordant
dizygotic pairs but not in discordant monozygotic pairs.
Comorbidity between cannabis dependence and MDD likely arises through
shared genetic and environmental vulnerabilities predisposing to both outcomes.
In contrast, associations between cannabis dependence and suicidal behaviors
cannot be entirely explained by common predisposing genetic and/or shared
environmental predispositions. Previously reported associations between early-onset
cannabis use and subsequent MDD likely reflect shared genetic and environmental
vulnerabilities, although it remains possible that early-onset cannabis use
may predispose to suicide attempt.