Researchers Find Cognitive Impairment in Heavy Marijuana Users

MARIJUANA March 1996

Two researchers in Massachusetts have found that heavy marijuana users test lower on memory, learning, and attention tests than light users (Harrison G. Pope, Jr., MD, and Deborah Yurgelun-Todd, PhD, "The Residual Cognitive Effects of Heavy Marijuana Use in College Students," Journal of the American Medical Association, February 21, 1996, p. 521-527).

Through a newspaper advertisement, researchers recruited 161 university students who had used marijuana in the past. The students were divided into two groups: those who had used marijuana at least 22 of the last 30 days (heavy users) and those who had used at most 9 days in the last 30 (light users). Most of the heavy users reported smoking at least 27 days in the past month, and most of the light users had smoked on no more than three days. Numbers of men and women in each category were relatively the same.

161 subjects reported to the McLean Hospital in Belmont, Massachusetts at 2:00 pm on the first day of the study. Researchers administered questionnaires and various tests. All subjects were urine tested. Reported heavy users testing negative, reported light users testing positive, and any subjects testing positive for another drug were excluded. 65 heavy users and 64 light users remained after other subjects were excluded for various reasons (inconsistency in reported levels of marijuana use, suspected use of marijuana during the test, or prior head injury or psychiatric illness). There were no significant differences in the two groups in verbal or intelligence scales as measured by the Verbal Intelligence Quotient test and self-reported SAT scores. Demographically, the two groups were similar, although the reported family income of the heavy users was slightly higher than the light users.

The remaining subjects remained in the hospital overnight and were instructed not to use marijuana. They were monitored through a one-way glass for the duration of the study. Monitored abstinence for at least 19 hours ensured that subjects were not under the influence of marijuana at the time of the cognitive tests.

The next day, subjects were given a number of tests to measure learning, attention, memory, verbal ability, intelligence, and sorting ability. Heavy users scored lower on attention, executive system functioning, and learning tests. Recall of recently-learned information remained the same between heavy and light users. Male heavy users scored lower than female heavy users on the tests, but the authors point out that this difference may be attributable to the males ingesting more of the drug in each smoking episode.

The researchers note that this experiment measured the residual effects of marijuana use. It did not measure whether the effects were due to the residual effects of marijuana remaining in the system or the effects of withdrawal, or the residual effects of the marijuana on the brain over time.

In an accompanying editorial, JAMA's editors caution that the study's findings should not be exaggerated (Editorial, "Does Heavy Marijuana Use Impair Human Cognition and Brain Function?" Journal of the American Medical Association, February 21, 1996, p. 560-561). Many studies should be undertaken to determine the long-term effects of marijuana. Such a project, the editors write, is important in light of surveys showing recent increases in use of the drug. The editors write that one key question arising out of the study is whether the heavy user impairment is a result of damage to the brain, the drug remaining in the bloodstream over a prolonged period of heavy use, or withdrawal. The editors suggest that recent developments in brain imaging be used to map possible differences in heavy and light users during cognitive tasks.

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