Early and frequent use of cannabis might increase the risks of depression and suicidal ideation, researchers suggest
“As cannabis use continues to be viewed with greater permissiveness, we need to carefully evaluate the relationship between various levels of cannabis use – from casual to heavy and problematic – and serious mental health outcomes like depression and suicidal thoughts and behaviours,” Arpana Agrawal of Washington University School of Medicine in St Louis, Missouri, said.
“We need to also examine all possible pathways that might lead to these associations, such as shared genetic factors, and exclude them before ascribing causal effects,” she said by e-mail.
To investigate, Agrawal and colleagues analysed data from 13,986 twins (6,181 monozygotic and 7,805 same-sex dizygotic) from three Australian twin studies: 1992 to 1993 (sample 1, consisting of 4,432 individuals), 1996 to 2000 (sample 2 – 6,255), and 2005 to 2009 (sample 3 – 3,299).
“Early use” of cannabis was defined as starting before age 18 for sample 1 and before age 17 for samples 2 and 3. “Frequent users” were those who had used cannabis 100 times or more. As reported in The Lancet Psychiatry, cannabis use was higher in samples 2 (59.8 per cent) and 3 (69 per cent) than in sample 1 (30.4 per cent).
The mean age at onset of cannabis use was higher in sample 1 (21 years old) but similar in samples 2 (about 19) and 3 (about 18). There was a correlation between early use and frequent use: 44 per cent of early users also reported frequent use and 49 per cent of frequent users also reported use from a young age.
A constellation of mechanisms link cannabis use to suicidal thoughts and behaviours
Similar to the cannabis findings, the age at onset of both suicidal ideation and suicide attempt was higher in sample 1 (about 24) than in samples 2 and 3 (about 20 in both).
Major depressive disorder, suicidal ideation and suicide attempt were moderately correlated. Almost everyone who reported a suicide attempt also reported ideation.
After adjustment for covariates, both early and frequent cannabis use were strongly associated with major depressive disorders and suicidal thoughts and behaviours. For instance, those who reported using cannabis at least 100 times were twice as likely to report suicidal ideation and suicide attempt.
Similarly, monozygotic twins who used cannabis frequently were more likely to report major depressive disorder cases (odds ratio, 1.98) and suicidal ideation (or, 2.47) than were their identical twin siblings who used cannabis less frequently. “Overall, these analyses suggest that early and frequent cannabis use are associated with major depressive disorder and suicidal thoughts and behaviours, even after accounting for key confounders” such as genetics, the authors conclude.
As next steps, the teamwill explore biological contributions to the potential links between heavy cannabis use and depression as well as suicidal thoughts and behaviours, according to Agrawal.
“For instance,” he said, “the endocannabinoid system, which regulates our ability to respond and adapt to stress, is a target for cannabis and also contributes to the development of certain aspects of low mood.”
Editorialist Laura Huckins of Mount Sinai’s Icahn School of Medicine in New York City said that finding a dose response between heavy cannabis use and suicidal behaviours “is a good indication that some changes in neurobiology might be occurring due to cannabis smoking, which then lead to an increase in suicidal behaviours”.
“The most important message is that a constellation of mechanisms link cannabis use to suicidal thoughts and behaviours,” she said by e-mail.
“In order to formulate effective policies for suicide prevention and improved mental health in the community, scientists, clinicians and policymakers should consider and address these mechanisms concurrently.”
“The psychiatric genetics community has recently identified a number of genetic variants associated with depression, as well as suicidal thoughts and behaviours,” she noted. “We also have elegant algorithms and software to test for genetic correlation between different traits [which] could be used to explore the shared genetic risk factors between cannabis use and suicidal thoughts/behaviours.”
“Another interesting avenue for researchers is to look at the environmental factors surrounding cannabis use,” she added. “The authors suggest that worsened societal outcomes resulting from cannabis use might lead to a higher risk of depression and suicidal thoughts/behaviours. A simple correlate of this is that reducing stigma and criminality around cannabis use [i.e., improving societal outcomes] should reduce the rate of depression and suicidal thoughts/behaviours in these individuals.”