Article by Angela Stelmakowich, Growth Op
Irish researchers say their findings that frequent or dependent cannabis use during adolescence could spur a two-point drop in IQ over time offers further insight into weed’s harmful neurological and cognitive effects on young people.
Carried out by researchers at RCSI (Royal College of Surgeons in Ireland) University of Medicine and Health Sciences and the Beaumont Hospital, a new paper detailing the systematic review and meta-analysis was published last week in Psychological Medicine.
Researchers did related searches of several databases from their respective inceptions until January 2020, identifying seven longitudinal studies, all of which had a baseline IQ score prior to starting cannabis use and another IQ score at follow-up. Involving 808 young people who used weed at least weekly for a minimum of six months compared to 5,308 young people who did not use cannabis, participants were followed up until age 18 on average, though one study continued until age 38.
“We found a significant effect for the association between frequent or dependent cannabis use in youth and IQ change,” compared to those who didn’t use marijuana, study authors write.
Researchers acknowledge that although the quality of the studies reviewed was moderate to high, statistical heterogeneity among studies was low. Combined, findings of the studies translate to “an average decline of approximately two IQ points following exposure to cannabis in youth,” they say.
“Further analysis suggested that this decline in IQ points was primarily related to reduction in verbal IQ,” adds a statement from the university. The Encyclopedia of Clinical Neuropsychology defines verbal IQ as the score of an individual’s overall verbal intellectual abilities and is a measure of acquired knowledge, verbal reasoning and attention to verbal materials.