A new UK study has found that occasional adolescent cannabis
use does not lead to poorer educational and intellectual performance, but that
heavy cannabis use is associated with slightly poorer exam results at age 16.
The results come from the Avon Longitudinal Study of Parents
and Children (ALSPAC, also known as "Children of the 90's") a long-term study
that follows the health of children born in the Bristol area (UK) in 1991 and
1992. The work is being presented at the annual congress of the European
College of Neuropsychopharmacology (ECNP) in Berlin.
The researchers analysed data from 2,612 children who had
their IQ tested at the age of 8, and again at the age of 15. These children's
examination results were then factored in via the National Pupil Database.At
the age of 15, each person in the study completed a survey on cannabis use. The
researchers then used regression analysis to look at how cannabis use affected
both intellectual and educational performance. A number of children could not
be included in the final analyses (for example because they had experienced a
head injury), leaving a total sample size of 2,235.
The researchers' two main points
use appeared to be associated with decreased intellectual performance. Cannabis
use was, however, highly correlated with other risky behaviours such as
alcohol, cigarette and other drug use. When the researchers took these other
behaviours into account, they found there was no relationship between cannabis
use and lower IQ at age 15.
* Heavier cannabis users (at least 50
times by age 15) however, did show marginally impaired educational abilities.
These children tended to have poorer exam results (3% lower) on compulsory
school exams taken at age 16, even after adjusting for childhood educational
performance, as well as alcohol, cigarette and other drug use.
Lead researcher, Claire Mokrysz, said, "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
"People often believe that using cannabis can be very
damaging to intellectual ability in the long-term, but it is extremely
difficult to separate the direct effects of cannabis from other potential
explanations. Adolescent cannabis use often goes hand in hand with other drug
use, such as alcohol and cigarette smoking, as well as other risky lifestyle
choices. It's hard to know what causes what- do kids do badly at school because
they are smoking weed, or do they smoke weed because they're doing badly? This
study suggests it is not as simple as saying cannabis is the problem."
"This is a potentially important public health message- the
belief that cannabis is particularly harmful may detract focus from and
awareness of other potentially harmful behaviours. However the finding that
heavier cannabis use is linked to marginally worse educational performance is
important to note, warranting further investigation."
Commenting ECNP Chair, Professor Guy Goodwin (Oxford) said,
"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
The researchers noted that the study has some limitations.
For example, cannabis use was self-reported, and the measure of IQ taken at age
15 was an abbreviated version of the standard Wechsler IQ test. Full details
can be found in the abstract (however please note that the abstract shows a
preliminary analysis; this may differ from the version which is currently being
prepared for publication and which is described above).