Frequent or dependent cannabis use in adolescence is associated with an almost 2-point decline in IQ over time, according to new research published in Psychological Medicine.
“Cannabis use in general is growing in prevalence in western countries, generally concurrent with legalization. There is little high quality evidence that cannabis is either safe or has significant medicinal properties outside perhaps rare juvenile onset epilepsies,” said study author Emmet Power, a clinical research fellow in the Royal College of Surgeons in Ireland.
The researchers conducted a meta-analysis of seven longitudinal studies involving 808 young people who frequently used cannabis and 5,308 young people who had used cannabis less than five times in their entire life. All of the studies included a baseline IQ score prior to starting cannabis use and another IQ score at follow-up. Most of the participants were followed up until age 18, although one study followed them until age 38.
Participants were considered to be frequent users if they had used the drug at least once a week for 6 months, had more than 25 reported lifetime uses, and/or had a diagnosis of cannabis dependency.
The researchers found declines of approximately two IQ points in those who use cannabis frequently compared to those who didn’t use cannabis. Further analysis suggested that this decline in IQ points was primarily related to reduction in verbal IQ.
“Adolescence and early adulthood are crucial periods for completing education and establishing career trajectories and social relationships for later in life and given the negative effects of cannabis use in this age group, reducing the prevalence of its use should remain a priority,” the researchers wrote in their study.
But the “mechanisms of this association remain unclear,” Power said.
The researchers mentioned several potential explanations for the link between adolescent cannabis use and reduced IQ, including “a developmental neurotoxicity mechanism, a social pathway influenced by deviancy and educational non-engagement, by residual effects of cannabis or by individual vulnerability factors such as reading ability in childhood or by genetic factors.” But additional research is needed to examine these factors.
“There are many remaining questions to be asked,” Power added. “Specifically, it would be important in future studies to investigate any dose-response relationship, longitudinal effects on executive functioning and whether residual impairment remains after use.”
The study, “Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies“, was authored by Emmet Power, Sophie Sabherwal, Colm Healy, Aisling O’ Neill, David Cotter and Mary Cannon.
