Young Stoners Run Risk of Lower IQ
Individuals repeatedly diagnosed with cannabis dependence during
young adulthood had noticeable declines in IQ scores by age 38,
especially when the heavy use started in their teens, researchers said.
A
small to medium decline in mean IQ between tests taken on the eve of
adolescence and again at age 38 was seen in those diagnosed at least
three times with cannabis dependence, according to Madeline Meier, PhD,
of Duke University, and colleagues, who reported on data from 1,037
participants in a New Zealand birth cohort.
In the small group of
participants who became cannabis dependent before age 18 -- a total of
23 cohort members -- the decline translated to an average of about 8
IQ points, whereas 14 participants who also showed heavy cannabis use
but only beginning in adulthood showed only a very small drop in
full-scale scores (
P=0.02), Meier and colleagues indicated online in
Proceedings of the National Academy of Sciences.
The
researchers also reported that the size of the mean decline increased
with the number of cannabis dependence diagnoses that participants had
received in five evaluations conducted from ages 18 to 38 (
P<0.0001 for trend).
"Collectively,
[the study's] findings are consistent with speculation that cannabis
use in adolescence, when the brain is undergoing critical development,
may have neurotoxic effects," Meier and colleagues wrote.
Study
participants were members of the Dunedin Multidisciplinary Health and
Development Study. It attempted to track all children born in Dunedin,
New Zealand, from April 1972 to March 1973 starting at age 3. At age 38,
the investigators had data on IQ testing and cannabis dependence
diagnoses on 874 cohort members.
Besides assessing cannabis use,
the five structured interviews in adulthood also elicited information on
use of other illicit drugs and alcohol. IQ tests were conducted
initially at ages 7 to 13 and again at age 38.
Among participants
included in the current analysis, 242 never reported cannabis use in the
structured interviews nor were they ever diagnosed with dependence; 479
indicated some use but never received a diagnosis; and 80, 35, and 38
had received one, two, or three or more dependence diagnoses in the
adult evaluations.
Baseline mean IQ scores were similar and close
to the standardized population average of 100 in these cannabis-use
subgroups. However, at age 38, the mean scores had diverged
considerably. The change from baseline was as follows (
P values not reported):
- Never used, never diagnosed: +0.80
- Used, never diagnosed: -1.07
- One diagnosis: -1.62
- Two diagnoses: -2.47
- Three or more diagnoses: -5.75
Meier
and colleagues reported other changes over time in "standard deviation
units," in which changes of 0.20, 0.50, and 0.80 should be considered
small, medium, or large, respectively.
For participants with three or more diagnoses, the change was -0.38 units.
The
researchers also stratified participants into two groups according to
whether, at a given interview, they reported using cannabis at least
once a week on average ("regular user").
Individuals classed as
regular users at least three times in the study also showed the largest
declines in IQ between tests (mean -5.23 IQ points, -0.35 standard
deviation units,
P value not reported), whereas little change was seen in those never reporting regular use.
Similar patterns were seen in IQ subdomain scores.
These
small to medium declines were also seen in this participant group in
tests of memory, processing speed, and executive function.
Meier
and colleagues sought to rule out potential confounding factors, taking
educational attainment and use of alcohol and other drugs into account.
When
they restricted their analysis to 278 participants who did not get
beyond high school, again those with three or more dependence diagnoses
showed medium-level declines in IQ (mean -0.48 standard deviation units)
whereas those in the never-used, never-diagnosed category had
essentially no change (mean -0.03 units,
P=0.0009 for trend).
The
pattern also held up when the researchers excluded participants with
persistent dependence on tobacco, alcohol, and "hard" drugs, and also
those with diagnoses of schizophrenia.
However, the strongest
relationships between persistent dependence and IQ decline applied to
those who began heavy use in adolescence. Meier and colleagues suggested
that, actually, the relationship between dependence and IQ decline may
be entirely driven by this group.
The 23 participants with
adolescent-onset dependence and at least three diagnoses overall showed a
mean decline of about 0.53 standard deviation units, compared with a
drop of about 0.13 among those with three diagnoses that all came during
adulthood (
P=0.02).
Nonsignificant trends toward greater
declines in IQ were also seen in those teen-onset dependence but fewer
total diagnoses of dependence.
"In fact, adult-onset cannabis
users did not appear to experience IQ decline as a function of
persistent cannabis use," Meier and colleagues wrote.
They
cautioned that their data did not prove that heavy cannabis use actually
caused the decline.
"There may be some unknown 'third' variable that
could account for the findings. The data also cannot reveal the
mechanism underlying the association between persistent cannabis
dependence and neuropsychological decline," they wrote.
They also acknowledged that their data relied on self-report of cannabis use, without confirmation from blood or urine testing.
But these caveats did not stop them from speculating on causal mechanisms and the implications of a causal relationship.
Meier
and colleagues noted that animal studies as well as theories of neural
development during adolescence support a neurotoxic effect of cannabis
in the young adult brain.
Moreover, they argued, their findings should inform public health programs.
"Prevention
and policy efforts should focus on delivering to the public the message
that cannabis use during adolescence can have harmful effects on
neuropsychological functioning, delaying the onset of cannabis use at
least until adulthood, and encouraging cessation of cannabis use
particularly for those who began using cannabis in adolescence," they
wrote.
Duke University, in a press release, was even less
circumspect in a press release headlined, "Adolescent pot use leaves
lasting mental deficits."
Its first sentence claimed that "the
persistent, dependent use of marijuana before age 18 has been shown to
cause lasting harm to a person's intelligence, attention and memory" in
the study.
The
Dunedin cohort study was funded by the New Zealand Health Research
Council. The current analysis was supported by the U.K. Medical Research
Council, the U.S. National Institutes of Health, and the Jacobs
Foundation.
No potential conflicts of interest were reported.
