major depressive disorder

A recent study looks at the probability of developing major depressive disorder (MDD) and suicidal ideation (thoughts and behaviours) with early and regular use of cannabis.


As demonstrated by a 30-year longitudinal study controlling for confounding variables like mental health characteristics, genetic liability, early family environment, and other familial risk factors, the use of cannabis even once a week has been associated with the new occurrences of suicidal ideation, especially in men.  Another study found that individuals who were dependent on cannabis were more likely to report suicidal thoughts and behaviours than their monozygotic (identical) and dizygotic (fraternal) twins who did not exhibit dependence on cannabis, suggesting that early, heavy or frequent cannabis use (or use resulting in abuse and dependence) might be related to major depressive disorder genetically, whereas correlations to suicidal ideation might be caused by non-genetic, environmental factors.

A recent study published in The Lancet Psychiatry aimed to further investigate this correlation by examining whether a lifetime history of cannabis use in addition to early-onset use and frequent use was associated with major depressive disorder, suicidal ideation, persistent ideation, ideation with a plan, and suicide attempt.

The study incorporates a large set of retrospective data from three samples of same-sex male and female twins (13,986 enrolled) drawn from 3 studies that had recruited twins from the Australian Twin Registry. Participants’ histories of cannabis use were assessed and recorded, and the respondents that met the criteria as described by the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) were clinically diagnosed with major depressive disorder.  The analyzed data suggested that the twin who used cannabis more frequently was more likely to report major depressive disorder and suicidal thoughts and behaviours compared with the twin who consumed cannabis less frequently. Related to early cannabis use, the data indicated a possible association with suicidal ideation.

Although this study revealed these associations, however, a causal relationship between cannabis, major depressive disorder, and suicidal ideation cannot yet be either inferred nor discounted.

Both animal and human studies have hypothesized that the endocannabinoid system may be an integral component in mood modulation, especially when stress is involved. The endocannabinoid system consists of a group of receptors in the brain involved in a wide range of physiological processes, such as appetite, pain-sensation, mood, and memory, as well as the facilitation of cannabis’s psychoactive effects.  One current theory proposes that frequent use of cannabis might result in modifications to the endocannabinoid system, resulting in a negative mood.  Additionally, regular cannabis consumption may increase one’s exposure to environmental factors (e.g. trauma) or outcomes (e.g. diminished life opportunities, other drug use) that might also increase the risk of major depressive disorder and suicidal ideation.

The prevention of early and frequent cannabis use may be a feasible strategy to improve the outcomes of individuals experiencing serious mental illnesses, such as major depressive disorder.  Further research is required to fully explain these associations with the hopes of applying these findings in a clinical setting.


Written by Jordyn Posluns, B.Sc. (Hons)

Reference: Agrawal A, Nelson EC, Bucholz KK, et al. Major depressive disorder, suicidal thoughts and behaviours, and cannabis involvement in discordant twins: a retrospective
cohort study. The Lancet. 2017 July 24.

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