The consistent consumption of cannabis over three decades doesn’t appear to significantly heighten the risk of mental health issues or significantly impair cognitive function, according to the results of a longitudinal study published in the Journal of Psychopathology and Clinical Science.
Scientists affiliated with the University of Minnesota and the University of Colorado studied over 4000 twins in the USA, beginning in 1994 and finishing in 2021. Participants were observed to have used cannabis on average 1 to 2 times a month over the 27-year duration of the study.
Although results showed no negative outcomes for the user in terms of their mental health, they did indicate that regular cannabis use over a lifetime could contribute towards an increased risk of developing cannabis use disorder, using illicit drugs, and a higher frequency of tobacco use.
The purpose of using twins in the study was to increase the accuracy of the results by allowing researchers to control influences from environmental and genealogical factors.
The design of the study proved to be important in this instance because after the results were analysed at an individual level the negative effect cannabis had on users appeared to increase. However, when a twin was compared with their co-twin and influential factors were considered the results showed a less harmful effect on users when observed over the entirety of the study.
“As expected, in individual-level analyses, cannabis use was significantly associated with almost all outcomes in the expected directions. However, when comparing each twin to their cotwin, which inherently controls for shared genes and environments, we observed within-pair differences consistent with possible causality in three of the 22 assessed outcomes: cannabis use disorder symptoms, frequency of tobacco use, and illicit drug involvement.” the study states.
In their conclusion, the authors emphasise the genetic and environmental factors they felt have a stronger influence than cannabis use in contributing to mental health issues and cognitive decline, “Broadly speaking, our results do not support a causal relationship between lifetime average cannabis frequency and most of the substance use, psychiatric, and psychosocial outcomes assessed here. Rather, genetic and familial confounding most likely explain the relationships between cannabis use and the negative outcomes associated with it. The lack of within-pair effects, or small effects for those existing within-pair differences, in our primary outcome suggest that cumulative cannabis use does not have large, or lasting effects on many psychosocial outcomes.”