People having been ingesting cannabis in different forms for centuries, and until it became illegal here in the United States in the mid 1920’s, it was used nationwide. Yet despite its popularity, our understanding of how cannabis affects users remains quite limited as a result of legal constraints that make research on it a difficult endeavor. Cannabis use is becoming increasingly widespread as many states take steps to decriminalize it, pass medical marijuana laws, and legalize its recreational use. In this blog we will take a look at four research studies that look at the relationship between cannabis use and mental health.

Cannabis Use among Youth

The first study we examine began in 1995 and followed 1,395 teenagers aged 14-17 in Munich Germany for a period of over ten years. At the beginning of the study, the prevalence of cannabis use among subjects was 19.3 % and the prevalence of cannabis use disorder (more heavy use causing disruption of function) was 2.6%. Ten years later, the cannabis use prevalence increased to 54.3% and cannabis use disorder to 13.7%. The study noted that the average onset of use was 15 years of age. The study also noted that certain mental health conditions such as bipolar disorder, other mood disorders and, to some extent, anxiety disorders contributed to the likelihood of increasing cannabis use and cannabis use disorder (1).

A second longitudinal study tracked cannabis use among 1,943 Australian youth over a period of fifteen years, from their mid-teens to their late twenties. Researchers surveyed participant cannabis use a total of 9 times: 5 times in adolescence and an additional 3 times in young adulthood. They found that “[r]egular (particularly daily) adolescent cannabis use is associated consistently with anxiety, but not depressive disorder, in adolescence and late young adulthood, even among regular users who then cease using the drug.” While they found a clear association, it is important to note that the causal relationship is not clear: are adolescents who experience anxiety self medicating with cannabis or is cannabis use causing anxiety? The results are striking regardless and warrant more research on the topic. (2)

Cannabis as Treatment for Existing Conditions

What about cannabis use in individuals with a pre-existing psychiatric disorder?

A meta-analysis of six studies with a total sample size of 2,391 and average follow up period of 3.9 years, looked at cannabis use in persons with a prior diagnosis of bipolar disorder. The study found that cannabis use can contribute to worsening of bipolar disorder symptoms and can increase the risk of developing additional manic episodes by three-fold. These findings strongly suggest that those with a history of bipolar disorder take caution with cannabis use. (3)

Another study looked at the link between cannabis use in teens and the risk for developing schizophrenia-type symptoms and/ or depression by age 26. The study divided the 1,037 subjects into three groups:

Group 1: Those who used cannabis at least 3 times by age 15

Group 2: Those who used cannabis by age 18

Group 3: Those who either never used cannabis or used it only once or twice by age 18 (control group)

The results showed that while there was no link between cannabis use and depression by age 26 in any of the groups, in the group that reported cannabis use by age 15, there was a four-fold increase in likelihood that they would experience psychotic symptoms by age 26. Yet when the researchers controlled for individuals who demonstrated psychotic spectrum symptoms by age 11, this association became so small that it was no longer statistically significant (4). While this study suggests the need for caution when it comes to adolescent cannabis use, the results as to the causal relationship between cannabis and psychosis is unclear.

More Research is needed

The available research is limited in scope and while we know that cannabis use is very common and starts early in life, we do not fully understand (1) what impact it may have on brain development, (2) how it may effect existing mental health conditions, or (3) the role it may play in development of new mental health disorders. The available research at the very least warrants that those with either a pre-existing mental health disorder or a family history of biploar and/or schizophrenia take serious caution when it comes to cannabis use.