Early Cannabis Use Is Independently Linked to Self-Harm, While Lifetime Use Shares Genetic Roots | Research

The takeaway

A twin study found that lifetime cannabis use and self-injury share underlying genetic and family factors, but early cannabis use before age 17 appears to have an independent, potentially causal connection to self-harm.

Read this if you want to understand whether cannabis use itself causes self-harm or whether both reflect shared underlying risk factors.

3.2x higher self-harm odds for early cannabis users, even in identical twins

What the researchers found

Researchers studied nearly 10,000 Australian twins to untangle whether cannabis use leads to nonsuicidal self-injury (NSSI) or whether both behaviors stem from shared genetic and environmental factors.

Lifetime cannabis use was associated with a 2.84 times higher odds of self-injury. However, when the researchers compared identical twins where one used cannabis and the other did not, this association disappeared. This means the link between lifetime cannabis use and self-injury is likely explained by shared genes and family environment rather than cannabis itself.

But the story changed for early cannabis use (before age 17). Even in identical twin pairs discordant for early use, the twin who started cannabis early had 3.2 times higher odds of self-injury. This suggests that starting cannabis use in adolescence carries a person-specific risk for self-harm that goes beyond shared genetics.

Why it matters

This is one of the most methodologically rigorous studies examining cannabis and self-harm because the twin design can separate genetic predisposition from potential causal effects. The finding that early-onset cannabis use carries independent risk even after controlling for genetics has direct implications for prevention efforts targeting adolescent cannabis use.

The numbers in context

Lifetime cannabis use: OR = 2.84 (95% CI 2.23-3.61). Early cannabis use: OR = 2.15 (95% CI 1.75-2.65). In discordant identical twins, early cannabis use: OR = 3.20 (95% CI 1.17-8.73). Sample: 9,583 twins plus 3,787 replication sample.

How the study worked

The study used data from 9,583 adult twins in the Australian Twin Registry. Researchers compared monozygotic (identical) twins discordant for cannabis use, which controls for shared genetics and family environment. Results were replicated in an independent sample of 3,787 female twins with temporal ordering of onset.

What this study cannot tell us

The discordant twin sample for early cannabis use was small, which limits the precision of the effect estimate. Self-injury and cannabis use were both self-reported. The study cannot identify the specific mechanisms by which early cannabis use might increase self-harm risk.

How to read the evidence

Twin design provides strong control for genetic and family confounds, with an independent replication sample. However, the discordant twin sample was small.

When this study was published

Published in 2016 using data from the Australian Twin Registry. Cannabis potency has increased substantially since most participants' adolescence.

The bigger picture

This study adds important nuance to the cannabis-mental health debate. It suggests that the link between cannabis use and self-harm is not straightforward. For most adult users, the connection appears to reflect shared vulnerability rather than a direct effect of cannabis. But for adolescents who start early, there may be a window of vulnerability where cannabis exposure itself contributes to self-harm risk.

Questions still open

What is it about early adolescent cannabis exposure that creates independent self-harm risk? Would the results differ with current high-potency cannabis products? Does the type or frequency of early use matter?

Common questions

Does cannabis cause self-harm?

It depends on timing. For adults, the link appears to be explained by shared genetics and family environment. But starting cannabis before age 17 may carry independent risk for self-injury beyond genetic predisposition.

Why does the age of first use matter?

Adolescent brains are still developing, and the twin study suggests that cannabis exposure during this window may create vulnerabilities to self-harm that go beyond the shared genetic factors that drive both behaviors in adulthood.

Read the original research

Cannabis Involvement and Nonsuicidal Self-Injury: A Discordant Twin Approach.

Journal of studies on alcohol and drugs, 77(6), 873-880

View on PubMed ↗

Citation

Few, Lauren R; Grant, Julia D; Nelson, Elliot C; Trull, Timothy J; Grucza, Richard A; Bucholz, Kathleen K; Verweij, Karin J H; Martin, Nicholas G; Statham, Dixie J; Madden, Pamela A F; Heath, Andrew C; Lynskey, Michael T; Agrawal, Arpana. (2016). Cannabis Involvement and Nonsuicidal Self-Injury: A Discordant Twin Approach.. Journal of studies on alcohol and drugs, 77(6), 873-880.