Longitudinal UK Study Shows No Relationship Between Moderate Adolescent Cannabis Use and IQ

A 20 year longitudinal study has recently found that moderate cannabis use in adolescence has no association with detrimental effects on educational and intellectual performance, although heavy cannabis use is associated with poorer exam results at age 16. These findings come from the Avon Longitudinal Study of Parents and Children (also known as “children of the 90’s”), a longitudinal study conducted in the UK that tracks the health of children born in the Bristol area in 1991 and 1992.

Researchers collected data from 2,612 children and tested their IQ at the age of 8 and later at the age of 15. The children’s data were compared with other students in the National Pupil Database, an archive of information about students in England. Once they reached the age of 15, each student in the study filled out a survey on cannabis use. Researchers then analyzed the data to see how cannabis use affected intellectual and school performance.

Initially, there appeared to be a relationship between cannabis use and lower intellectual performance. But researchers also noted that cannabis use was highly related to other risky behaviours such as the use of alcohol, cigarettes and other drugs. After accounting for these factors researchers found no relationship between cannabis use and lower IQ scores at age 15. The results of the study should be taken with a grain of salt, however, as researchers noted limitations of the study; measures of cannabis use were dependent on the children’s self reported use, and the the IQ test taken at age 15 was an abbreviated version of the standard Wechsler IQ test.

Lead researcher, Claire Mokrysz of the University College London summarized, “Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors- particularly cigarette and alcohol use. This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behaviour and personal history typically associated with cannabis use, rather than cannabis use itself.”

The point that should be taken from this study, is that it is impossible to separate cannabis use from the use of other drugs like alcohol and cigarettes, as well as other risky behavior. It’s a bit like the chicken and the egg question: do youth do poorly because they smoke cannabis, or do they smoke cannabis because they do poorly?

ECNP Chair, Professor Guy Goodwin of Oxford explained, “This is a potentially important study because it suggests that the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself.”

The study did find that heavy cannabis use, (defined in the study as smoking at least 50 times by age 15) did have impaired educational performance with test scores three percent lower than average, even after accounting for childhood educational performance, as well as alcohol, cigarettes and other drug use.

Claire Mokrysz said that “the finding that heavier cannabis use is linked to marginally worse educational performance is important to note, warranting further investigation.” Heavy cannabis use in youth may indeed impair learning later in life, but further research is yet to reveal the nature of this relationship. Until then it may be safe to say that letting young children smoke large quantities of weed at a young age probably isn’t the best thing for their development.