CONTACT: JENNIFER CRONIN
2130 Medical Laboratories
Iowa City IA 52242
(319) 335-9917; fax(319) 335-8034
Release: March 29, 2000
UI studies: Frequent marijuana use may affect brain function but not structure
IOWA CITY, Iowa Recent University of Iowa Health Care studies indicate
that some people who frequently use marijuana have substantially lower blood
flow to certain parts of their brains; however, smoking the illicit drug does
not affect brain size or structure.
"Although marijuana is the most widely used illicit drug, surprisingly
little attention has been given to the impact of frequent marijuana use on
the structure or functioning of the human brain," said Robert I. Block,
Ph.D., UI associate professor of anesthesia and lead investigator for the
studies. "We wanted to examine these questions."
The UI findings appear in separate articles in two recent issues of NeuroReport.
"The question of whether marijuana use produces harmful effects is
important because marijuana is the most popular illegal drug, and also because
there is a lot of interest in its potential value as a medicine," Block
said. "Many people believe that harmful effects have not been proven
as clearly with marijuana as with most other illegal drugs. Our research showed
some fairly dramatic changes in brain blood flow in some frequent marijuana
users, even after the immediate effects of smoking had worn off."
In one study, Block and his colleagues used positron emission tomography
(PET) scanning techniques to determine whether frequent marijuana use affected
brain function. They compared regional cerebral blood flow in 17 current,
frequent young adult marijuana users with 12 comparable, control subjects
who did not use marijuana. During the study, the subjects were lying quietly
and did not perform any test and were not given any specific directions about
what to think about. The marijuana users, who were using marijuana seven or
more times weekly on average, were required to abstain from using the drug
for at least 26 hours prior to the scan to eliminate short-term effects of
The scan results showed that the marijuana users had up to 18 percent lower
blood flow than controls in a large region -- over a cubic inch of brain tissue
-- at the back of the brain, in the posterior cerebellum. There was very little
effect of marijuana use anywhere else in the brain.
Changes in brain blood flow usually correspond to changes in brain activity,
so diminished blood flow indicates altered brain function in some frequent
"The idea that frequent marijuana use impairs mental abilities is still
controversial, but several recent studies, including our own, support such
an association," Block said. "Although the cerebellum was traditionally
thought mainly to be involved in controlling movements, more recent evidence
has shown that it also plays a role in memory, attention, and other mental
abilities. Some cognitive effects of marijuana use may be related to this
lower activity of the posterior cerebellum."
Block said that in past studies he and his colleagues have found changes
in blood flow in several parts of the brain, including but not limited to
the cerebellum, when frequent marijuana users perform memory and attention
"The most important next step would be to see if these changes in brain
function persist for a long time after a person stops using marijuana,"
Block said. "We had our subjects stop for at least a day, to eliminate
short-term effects of smoking. But it's possible that some of the brain changes
that we are seeing are withdrawal effects. If they go away after a few days
of abstinence, they would be less serious than if they persist for weeks or
months, or longer, after a person stops using marijuana."
In the other study published in NeuroReport, Block and his colleagues wanted
to establish what effects frequent marijuana use had on brain tissue volume
and composition. The researchers, using magnetic resonance imaging (MRI),
measured brain volume and composition of 18 frequent marijuana users and 13
non-using control subjects.
Automated techniques were used to measure the volume of the brain as a whole
and its major regions including, for most regions, separate measures of two
major tissue types: gray matter and white matter. The marijuana users showed
no evidence of overall or regional changes in volumes of brain tissue.
"In the 1970s, a scientific paper was published describing cerebral
atrophy in young marijuana users," Block said. "The paper was widely
publicized and quoted for some time regarding the harmful effects of marijuana,
but other researchers did not find any cerebral atrophy. Neuroimaging techniques
have improved tremendously over the years, and we wanted to see whether these
improved techniques would show decreases in brain tissue in any parts of the
marijuana users' brains. We found no evidence of atrophy or any other harmful
effects of frequent marijuana use on brain structure."
Although the study did not show any harmful effects, "these findings
don't give marijuana a clean bill of health," Block said. "Anatomical
abnormalities might occur at a microscopic level that cannot be detected by
MRI, and MRI abnormalities might be observed in individuals who used marijuana
for longer periods than the people we studied."
On average, the marijuana users in the test had used the drug for about
Block, who has led a number of marijuana investigations, will be one of
the speakers at a conference titled "Medical Marijuana: Science-Based
Clinical Applications." The conference, to be April 6-8 at the UI, is
the first national conference to educate health care professionals and researchers
on the therapeutics of marijuana.
A conference brochure containing a full list of speakers and panels, as
well as a registration form, can be obtained at the Web site http://www.drugsense.org/ncct,
or by calling the UI Center for Conferences and Institutes at (800) 551-9029
or (319) 335-4141.
University of Iowa Health Care describes the partnership between the
UI College of Medicine and the UI Hospitals and Clinics and the patient care,
medical education and research programs and services they provide.