Teenagers
who frequently smoke marijuana may be setting themselves up for
declines in intelligence and mental function that persist well into
adulthood, new research suggests
In
a decades-long study of more than 1,000 New Zealanders, researchers
found that adolescents who used marijuana at least four days per week
lost an average of eight IQ points between the ages of 13 and 38—a
pattern not seen among people who began smoking heavily only in
adulthood
Heavy pot smokers tended to show deficits in memory,
concentration, and overall brainpower in relation to their peers, but
these problems were more pronounced—and seemingly more lasting—among
those who picked up the habit as teens, the study found.
Individuals
who smoked heavily in adolescence had consistently lower IQs at age 38,
even if they’d cut back in the previous year. By contrast, the IQ of
the relative latecomers to marijuana was more closely linked with how
much pot they’d smoked recently.
“The effect of persistent
cannabis use on intellectual functioning is really confined to
adolescents, [which] suggests that adolescents, in particular, are
vulnerable to the effect of cannabis,” says lead author Madeline H.
Meier, Ph.D., a psychologist and postdoctoral researcher at Duke
University, in Durham, North Carolina.
Related links:
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An
eight-point decline in IQ isn’t negligible, the authors say. Previous
research suggests a drop in intelligence of that magnitude could, for
instance, affect a person’s long-term career prospects, job performance,
and income.
It’s reasonable to suspect that still-growing teen
brains might be especially sensitive to the cumulative effects of
marijuana, says Jeffrey Brosco, M.D., a professor of pediatrics at the
University of Miami Miller School of Medicine.
“In the developing
brain, neurons are growing and changing, synapses are forming,” says
Brosco, who was not involved in the study. “When there’s a lot of change
in any part of the body, particularly the brain, that usually means
it’s more vulnerable to environmental influences.”
The new study,
which was published today in the Proceedings of the National Academy of
Sciences, doesn’t prove that marijuana use directly impairs
intelligence. It does, however, provide some of the strongest evidence
to date of a cause-and-effect relationship.
The study
participants underwent IQ tests when they were 13—before they’d ever
tried marijuana—and again as adults, which provided the researchers a
before-and-after snapshot of their intelligence. Earlier studies that
found a link between mental ability and pot smoking have generally
looked at a single point in time, raising the possibility that low IQ
increases the likelihood of marijuana use, rather than vice versa, Meier
says.
But Meier and her colleagues weren’t able to entirely rule
out alternative explanations for the IQ declines seen in the study. For
instance, although they controlled for alcohol and drug use, they
focused on full-blown dependence (as opposed to more casual use) and
therefore may have underestimated the effects of teen drinking
“It’s
hard to be dependent as a teenager, so you worry about [whether] you
can be sure it’s the cannabis,” Brosco says. “Alcohol is well known to
be a neurotoxin.”
Only 5% of the study participants began smoking
marijuana regularly before age 18, and it’s not clear from the findings
whether less-frequent users might experience similar declines in IQ and
mental function. More research will be needed to determine the minimum
dosage of marijuana associated with these problems, the authors say.
