Cannabis Dependence and Suicide Risk: What Twin Studies Reveal About Shared Genetics | Research

The takeaway

A twin study found cannabis-dependent individuals had 2.5-2.9 times higher odds of suicidal thoughts and attempts than their non-dependent co-twin, and this association could not be fully explained by shared genetics.

Read this if you want to understand the relationship between cannabis dependence, depression, and suicide risk.

2.5-2.9x higher suicide risk in cannabis-dependent twins vs their non-dependent co-twin

What the researchers found

Among 277 twin pairs discordant for cannabis dependence, the cannabis-dependent twin had 2.5 to 2.9 times higher odds of suicidal ideation and suicide attempt compared to their non-dependent co-twin. This association persisted even when controlling for shared genetic and environmental factors, suggesting cannabis dependence itself may contribute to suicidal behavior.

In contrast, the association between cannabis dependence and major depressive disorder was found only in dizygotic twins (who share ~50% of genes), not in monozygotic twins (who share 100%), suggesting shared genetics explained the depression-cannabis link. Early cannabis use (before age 17) was associated with 3.5 times higher odds of subsequent suicide attempt but not depression or suicidal ideation.

Why it matters

By comparing twins discordant for cannabis dependence, this study could control for shared genetic and environmental factors in a way that observational studies cannot. The finding that the suicide link persisted while the depression link did not suggested fundamentally different causal pathways for these outcomes.

The numbers in context

277 twin pairs discordant for cannabis dependence. 311 pairs discordant for early cannabis use. Suicidal ideation OR: 2.5-2.9. Early-onset suicide attempt OR: 3.5 (95% CI 1.4-8.6). Median age: 30 years.

How the study worked

This was a cross-sectional survey of 277 same-sex twin pairs discordant for cannabis dependence and 311 pairs discordant for early-onset cannabis use (before age 17) from a general population twin registry. DSM-IV lifetime diagnoses of MDD, suicidal ideation, and suicide attempt were assessed by self-report.

What this study cannot tell us

Cross-sectional design limits causal inference despite the twin design. Self-reported diagnoses may not match clinical assessment. The study examined cannabis dependence rather than use, so findings may not apply to non-dependent users. Twin designs assume equal environmental sharing between MZ and DZ twins.

How to read the evidence

This is a large twin study from a general population registry with discordant pair analysis, providing strong evidence for causal inference.

When this study was published

Published in 2004. The cannabis-suicide link has been investigated further in subsequent longitudinal studies.

The bigger picture

This study contributed to the growing evidence that cannabis dependence may independently increase suicide risk, beyond what can be explained by shared genetic vulnerability. This finding has clinical implications for screening and intervention among people with cannabis use disorders.

Questions still open

What mechanism might link cannabis dependence to suicidal behavior independently of depression? Does treating cannabis dependence reduce suicide risk? Is the early-onset suicide risk specific to cannabis or common to early substance use generally?

Common questions

Does cannabis cause depression?

This twin study suggested that the link between cannabis dependence and depression was largely explained by shared genetics rather than cannabis causing depression. However, the link between cannabis dependence and suicidal behavior appeared to involve more than just shared genes.

Does starting cannabis young increase suicide risk?

Using cannabis before age 17 was associated with 3.5 times higher odds of later suicide attempt in this study, even after controlling for shared twin factors. However, this did not hold for depression or suicidal ideation alone.

Read the original research

Major depressive disorder, suicidal ideation, and suicide attempt in twins discordant for cannabis dependence and early-onset cannabis use.

Archives of general psychiatry, 61(10), 1026-32

View on PubMed ↗

Citation

Lynskey, Michael T; Glowinski, Anne L; Todorov, Alexandre A; Bucholz, Kathleen K; Madden, Pamela A F; Nelson, Elliot C; Statham, Dixie J; Martin, Nicholas G; Heath, Andrew C. (2004). Major depressive disorder, suicidal ideation, and suicide attempt in twins discordant for cannabis dependence and early-onset cannabis use.. Archives of general psychiatry, 61(10), 1026-32.