Twin study finds frequent cannabis use linked to depression and suicidal thoughts even after accounting for shared genetics | Research

The takeaway

In identical twins where one used cannabis frequently and the other did not, the frequent user was about twice as likely to have major depression and 2.5 times as likely to report suicidal thoughts.

Read this if you want to understand whether the link between cannabis and depression might be causal rather than coincidental.

Frequent cannabis use: 2x depression risk and 2.5x suicidal ideation risk within identical twin pairs

What the researchers found

Among nearly 14,000 twins from the Australian Twin Registry, the identical twin who used cannabis frequently (100+ times) was significantly more likely to report major depressive disorder (odds ratio 1.98) and suicidal ideation (odds ratio 2.47) compared to their genetically identical co-twin who used less.

These associations persisted even after adjusting for early alcohol and nicotine use, early mood symptoms, conduct disorder, and childhood sexual abuse. Because identical twins share 100% of their genes and their early family environment, this design helps rule out the possibility that shared genetic or environmental factors explain the link.

For early cannabis use (regardless of frequency), the association with suicidal ideation was present in dizygotic twins, and the monozygotic estimate was consistent though not statistically significant on its own.

Why it matters

The twin design is one of the strongest tools available for separating the effects of a behavior from the genetic predisposition toward that behavior. The finding that frequent cannabis use is linked to depression and suicidal thoughts even within identical twin pairs suggests that the association cannot be fully explained by shared genetic liability.

The numbers in context

Frequent cannabis users (100+ times) versus their identical twins: OR 1.98 (95% CI 1.11-3.53) for major depression, OR 2.47 (95% CI 1.19-5.10) for suicidal ideation. Sample: 13,986 twins across three study waves.

How the study worked

Logistic regression analysis of retrospective data from same-sex twin pairs drawn from three waves of the Australian Twin Registry (1992-2009). Researchers identified monozygotic and dizygotic twin pairs discordant for cannabis use and compared their rates of depression, suicidal ideation, suicide plans, and suicide attempts. The combined sample included 13,986 twins (6,181 monozygotic and 7,805 dizygotic).

What this study cannot tell us

Retrospective self-report data may be subject to recall bias. The study cannot determine directionality (whether cannabis led to depression or vice versa). Cannabis frequency was measured as a lifetime count, not continuous use. Cultural and policy contexts in Australia may differ from other countries. The confidence intervals for the monozygotic associations were wide.

How to read the evidence

Large twin study published in The Lancet Psychiatry with nearly 14,000 participants. The discordant twin design provides strong control for genetic and environmental confounding.

When this study was published

Published in 2017. Twin studies continue to be used to disentangle cannabis-mental health associations.

The bigger picture

The debate over whether cannabis causes depression or whether depressed people simply use more cannabis has been difficult to resolve. This twin study design, while not a randomized experiment, is among the strongest observational evidence available. The results suggest that frequent cannabis use may contribute to depression and suicidal thinking through individual-specific pathways rather than shared predisposition alone.

Questions still open

What are the individual-specific pathways through which frequent cannabis use might increase depression risk? Would the same pattern hold in twin samples from different countries or ethnic backgrounds? Does the type or potency of cannabis matter, or is frequency the key factor?

Common questions

Does this prove cannabis causes depression?

Not definitively. The twin design controls for shared genetics and early environment, which strengthens the case that the association is not purely due to predisposition. However, it cannot rule out all possible confounders, and the retrospective design means the temporal sequence is uncertain.

What counts as "frequent" cannabis use in this study?

Frequent use was defined as lifetime use of 100 or more times. Between 16% and 22% of cannabis users in the sample met this threshold, depending on the study wave.

Read the original research

Major depressive disorder, suicidal thoughts and behaviours, and cannabis involvement in discordant twins: a retrospective cohort study.

The lancet. Psychiatry, 4(9), 706-714

View on PubMed ↗Publisher’s paper ↗

Citation

Agrawal, Arpana; Nelson, Elliot C; Bucholz, Kathleen K; Tillman, Rebecca; Grucza, Richard A; Statham, Dixie J; Madden, Pamela Af; Martin, Nicholas G; Heath, Andrew C; Lynskey, Michael T. (2017). Major depressive disorder, suicidal thoughts and behaviours, and cannabis involvement in discordant twins: a retrospective cohort study.. The lancet. Psychiatry, 4(9), 706-714. https://doi.org/10.1016/S2215-0366(17)30280-8