The young adults who volunteered for the study were not dependent on pot, nor did they show any marijuana-related problems.
"What
we think we are seeing here is a very early indication of what becomes a
problem later on with prolonged use," things like lack of focus and
impaired judgment, said Dr. Hans Breiter, a study author.
Longer-term
studies will be needed to see if such brain changes cause any symptoms
over time, said Breiter, of the Northwestern University Feinberg School
of Medicine and Massachusetts General Hospital.
Previous
studies have shown mixed results in looking for brain changes from
marijuana use, perhaps because of differences in the techniques used, he
and others noted in Wednesday's issue of the Journal of Neurosciences.
The
study is among the first to focus on possible brain effects in
recreational pot smokers, said Dr. Nora Volkow, director of the National
Institute on Drug Abuse. The federal agency helped pay for the work.
She called the work important but preliminary.
The
20 pot users in the study, ages 18 to 25, said they smoked marijuana an
average of about four days a week, for an average total of about 11
joints. Half of them smoked fewer than six joints a week. Researchers
scanned their brains and compared the results to those of 20 non-users
who were matched for age, sex and other traits.
The
results showed differences in two brain areas associated with emotion
and motivation — the amygdala and the nucleus accumbens. Users showed
higher density than non-users, as well as differences in shape of those
areas. Both differences were more pronounced in those who reported
smoking more marijuana.
Volkow
said larger studies are needed to explore whether casual to moderate
marijuana use really does cause anatomical brain changes, and if so,
whether that leads to any impairment.
The current work doesn't determine whether casual to moderate marijuana use is harmful to the brain, she said.
Murat
Yucel of Monash University in Australia, who has studied the brains of
marijuana users but didn't participate in the new study, said in an
email that the new results suggest "the effects of marijuana can occur
much earlier than previously thought." Some of the effect may depend on a
person's age when marijuana use starts, he said.
Another
brain researcher, Krista Lisdahl of the University of
Wisconsin-Milwaukee, said her own work has found similar results. "I
think the clear message is we see brain alterations before you develop
dependence," she said.
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