Tue Mar 5, 5:37 PM ET
By Amy Norton
NEW YORK (Reuters Health) - Long-time, heavy marijuana users
may eventually see their memory and attention span go up in
smoke, new research suggests.
Investigators found that, among pot smokers seeking treatment
for marijuana dependence, long-time users performed more
poorly on tests of memory and attention than shorter-term
users and non-users.
The findings show that over time, marijuana smoking can cause
intellectual impairments that "endure beyond the period of
intoxication" and worsen the longer a person uses the drug,
the study authors report in the March 6th issue of The Journal
of the American Medical Association.
But another researcher not involved in the study pointed to
shortcomings in the work that he says make it tough to draw
In an accompanying editorial, Dr. Harrison G. Pope, Jr. notes
that marijuana users who seek drug treatment do not
necessarily reflect users in general, since these individuals
may have other health issues such as anxiety or depression.
In the study, researchers led by Dr. Nadia Solowij of the
University of New South Wales in Sydney, Australia studied 102
pot-smoking Americans and 33 non-users. Marijuana users
typically smoked every day, with long-time users doing so for
an average of 24 years. Shorter-term users had smoked for
about 10 years, on average. The vast majority said that they
currently were not using other drugs, or did so only
Results of the mental functioning tests--taken after at least
12 hours of abstinence--showed that long-time users performed
less well than shorter-term users and non-users.
"We found that long-term users had problems with learning,
storage of learned information and retrieval of information
from memory," Solowij told Reuters Health.
This does not mean the drug caused brain damage in these
cases, she said, explaining that the impairments seen in
long-time users were "relatively subtle."
Still, Solowij noted, the deficits could affect daily
functioning--hindering, for example, a person's ability to
study or remember an item he or she just read.
But although such deficits, if prolonged or irreversible,
would be of "grave concern," other studies have found no such
impairments in long-time marijuana users, according to Pope, a
researcher at Harvard Medical School and McLean Hospital in
Pope and his colleagues recently found "virtually no
significant differences" between heavy marijuana users and
non-users on a battery of neuropsychological tests.
Part of the difficulty in sizing up the impact of marijuana is
weeding the drug's effects out from the "background noise" of
other factors, like psychiatric problems and abuse of other
drugs, according to Pope. In this study, nearly half of the
long-time marijuana users had in the past regularly used or
abused alcohol or other drugs.
But there are also plausible biological reasons for why
sustained marijuana use could affect things like memory.
Solowij noted that the brain receptors the drug acts on exist
in large numbers in regions involved in memory. Over the
years, she said, marijuana exposure might change the way these
receptors and other brain chemicals operate.
According to Pope, it seems almost certain that marijuana
produces short-lasting mental deficits, but whether they
endure or worsen over time is still unclear.
Also unknown is whether any impairments are reversed after a
person stops smoking pot, Solowij pointed out.
"We do not yet know whether the impairments recover after
stopping or reducing (marijuana) use," she said. "We are
currently investigating this question."
SOURCE: The Journal of the American Medical Association