“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 behaviors,” Dr. Arpana Agrawal of Washington University School of Medicine in St. Louis, Missouri, told Reuters Health.
“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 email.
To investigate, Dr. Agrawal and colleagues analyzed 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, online July 24, cannabis use was higher in samples 2 (59.8%) and 3 (69%) than in sample 1 (30.4%).
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% of early users also reported frequent use, and 49% of frequent users also reported use from a young age.
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 behaviors. For instance, those who reported using cannabis at least 100 times were twice as likely to report suicidal ideation and suicide attempt, compared with those who reported less frequent use.
Similarly, monozygotic twins who used cannabis frequently were more likely to report major depressive disorder (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 behaviors, even after accounting for key confounders” such as genetics, the authors conclude.
As next steps, the team will explore biological contributions to the potential links between heavy cannabis use and depression as well as suicidal thoughts and behaviors, according to Dr. 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 Dr. 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 behaviors “is a good indication that some changes in neurobiology might be occurring due to cannabis smoking, which then lead to an increase in suicidal behaviors.”
“The most important message is that a constellation of mechanisms link cannabis use to suicidal thoughts and behaviors,” she told Reuters Health by email. “In order to formulate effective policies for suicide prevention and improved mental health in the community, scientists, clinicians and policy makers should consider and address these mechanisms concurrently.”
“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/behaviors. 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/behaviors in these individuals.”