A 2012 study linking persistent cannabis use to
neuropsychological decline has been discredited.
Image: shutterstock
In a study published in the journal PNAS in 2012 , scientists
from Duke University in the US reported that cannabis had
a neurotoxic effect on the adolescent brain, causing a loss
of up to 8 IQ points in the heaviest users. They found that
IQ, learning, memory, and executive functions all declined
in heavy users who started smoking before the age of 18.
The research was part of a longitudinal study of 1,037 New
Zealand children born between 1972 and 1973. Health,
intelligence and behaviour measures were taken
periodically on all participants, most recently at the age of
38.
The scientists warned that there may be other explanations
for their findings and that they could not "definitively attest
to whether this association [between persistent cannabis
use and IQ decline] was causal". The study did rule out
factors such as years of education, schizophrenia, hard-
drug and alcohol dependence, but did not account for other
relevant factors such as childhood trauma.
A study published in PNAS six months after the original
research questioned the methodology, saying: "Although it
would be too strong to say that the results have been
discredited, the methodology is flawed and the causal
inference drawn from the results premature."
The researchers argued that if you take into account the
effects of socioeconomic status on IQ, the decline in IQ due
to cannabis use is overestimated by the previous study -
and could in fact be zero.
A new study by the University College London has now
shed further light on the Duke University findings. This
study drew on a larger sample of adolescents; 2,612 UK
children born between 1991 and 1992. They found that
heavy cannabis use was in no way linked to IQ decline,
although alcohol use was strongly correlated with IQ loss
in eight to 15 year olds.
There was some evidence that cannabis use impacted
school test scores: the heaviest users scored 3 per cent
lower on exams at age 16. This mirrors other research,
which has linked heavy marijuana use to poor academic
performance and low school completion rates in teenagers.
However, a new study in California where cannabis was
recently legalised show that legalising the drug leads to no
increase in problematic adolescent behaviour such as
crime, drug overdose, driving under the influence, or school
dropout rates. This indicates that even if negative health
effects are a consequence of smoking cannabis, prohibition
might not be the best option.

But it kills short term memory cells thereby limiting intake of knowledge.