Long-Term Marijuana Use Tied to
Worse Verbal Memory in Middle Age
As marijuana becomes more accessible to young and
old alike in the U.S., researchers warn that long-term use of the drug may
cause lasting harm to at least one type of brain function.
A new
study based on following thousands of young adults into middle age finds that
long-term marijuana use is linked to poorer performance on verbal memory tests,
but other areas of brain function do not appear to be affected.
“We did
not expect to find such a consistent association with verbal memory for chronic
exposure to marijuana,” especially since the link held even when other factors
like cigarette smoking, alcohol use and other behavioral factors associated
with marijuana use were accounted for, said lead author Dr. Reto Auer of the
University of Lausanne, Switzerland.
Auer and
colleagues analyzed data from a 25-year U.S. study of young adults, which
included repeated measures of marijuana exposure over time and a standardized
test of verbal memory, processing speed and executive function in year 25.
Almost 3,500 participants completed the standardized tests.
At the
beginning of the study period in the 1980s, participants were 18 to 30 years
old and more than 80 percent reported past marijuana use. Just 12 percent
continued to use marijuana into middle age, according to the results in JAMA
Internal Medicine.
Researchers
found that as past years of marijuana use increased, verbal memory scores
decreased. In practical terms, the results meant that for every additional five
years of exposure, 50 percent of marijuana users would remember one less word
from a list of 15 tested words.
“Recreational
marijuana users use it to get high, to benefit from the transient change it
produces,” Auer told Reuters Health by email. “But this transient effect might
have long term consequences on the way the brain processes information and
could also have direct toxic effects on neurons.”
But, he
said, it is unclear from this observational study if lower verbal memory is a
cause or a consequence of marijuana use.
The study
only included self-reported marijuana use and did not employ brain imaging to
measure structural changes, the authors note. “Unfortunately,
as with all recreational drug exposures, it would not be feasible to perform a
long term randomized controlled trial to verify causation,” Auer said. “We are
left with trying to do the best we can in epidemiological studies and adjust
for potential confounders to estimate the potential causal effect of marijuana
on health outcomes.
Several
studies have found that cannabis users who have used daily over years, and
especially those who have used daily for decades, perform worse on various
cognitive tasks than their peers who have either not used cannabis or have used
it less frequently, for shorter periods, then stopped, said Wayne Hall, of the
University of Queensland, Australia, who coauthored a related commentary.
“People
who used occasionally in their 20s and discontinue as most cannabis users do,
are at low risk of developing any cognitive impairment,” Hall told Reuters
Health by email.
“But
cannabis is a drug, and like all drugs, it can harm users when used in
particular ways,” Hall said. “This message needs to be communicated to all
cannabis users and especially those in U.S. states where medical or
recreational cannabis use by adults is now legal.”
The new
results did not include information on how the marijuana was consumed, only on
the number of days of exposure in the month before each exam, Auer said.
All
marijuana users and the overall population interested in knowing the health
consequences of marijuana should have access to balanced, high quality
information from independent researchers, but no study has repeatedly measured
brain structure and marijuana use over time, Auer said.
“There is
a severe lack of well performed studies on the topic, especially considering
the proportion of the population who have been exposed, or are exposed to
marijuana,” he said.
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