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National Institute on Drug Abuse - NIDA NOTES
Research Advances
Volume 11, Number 3
May/June 1996

Studies Show Cognitive Impairments Linger in Heavy Marijuana Users

By Robert Mathias, NIDA NOTES Staff Writer

Students who smoke marijuana heavily may be limiting their ability to learn, according to a NIDA-funded study. The study found that college students who used marijuana regularly had impaired skills related to attention, memory, and learning 24 hours after they had last used the drug. The finding supports the results of previous NIDA-funded research that reported that adults who were chronic heavy marijuana users showed residual impairment in cognitive abilities a day after they had last used marijuana.

"Now we know that for students who smoke marijuana heavily, the ability to learn is affected not just while they are high, but for at least a day after," says NIDA Director Dr. Alan I. Leshner. Together with other NIDA-funded research that has shown a marked increase in daily marijuana use among young people in recent years, this finding underlines the importance of the Marijuana Use Prevention Initiative launched by Health and Human Services Secretary Dr. Donna Shalala last year, Dr. Leshner says. NIDA is playing a leading role in the initiative by providing science-based information to educate the public about the consequences of marijuana use. (For more information, see NIDA NOTES, November/December 1995, Marijuana Conference Advances HHS Secretary's Marijuana Initiative and March/April 1996, Facts About Marijuana and Marijuana Use.)

Using the Wisconsin Card Sorting Test, shown here, Dr. Harrison Pope, Jr. (left)
found that 24 hours after they stopped using the drug, heavy marijuana
smokers made a greater number of errors than light users made in sorting cards
into different categories.

Regular heavy marijuana use compromises the ability to learn and remember information primarily by impairing the ability to focus, sustain, and shift attention, says Dr. Harrison Pope, Jr., of McLean Hospital in Belmont, Massachusetts, who directed the recent study. Noting that the actual ability to recall information remains relatively unaffected, Dr. Pope says, "If you could get heavy users to learn an item, then they could remember it; the problem was getting them to learn it in the first place."

In the study conducted among college students, Dr. Pope and Dr. Deborah Yurgelun-Todd, also of McLean Hospital, tested the cognitive functioning of 65 heavy cannabis users, most of whom had smoked marijuana at least 27 out of the previous 30 days. The researchers compared the heavy users' cognitive functioning to that of a comparison group of 64 light users, most of whom had smoked marijuana on no more than 3 of the previous 30 days. Heavy users ranged in age from 18 to 24 years and light users from 18 to 28 years. The two groups were similar demographically and had comparable numbers of men and women. Subjects in both groups had smoked marijuana for at least 2 years, and none had smoked regularly for more than a decade.

To ensure that the subjects did not smoke marijuana or use other illicit drugs or alcohol during the study, researchers monitored them for 19 to 24 hours. Then the subjects performed a battery of standard tests designed to assess their ability to pay attention, learn, and recall new information. The tests indicated that heavy marijuana users had more difficulty than light users in sustain-ing and shifting attention and hence in registering, organizing, and using information. Heavy users exhibited these cognitive deficits by being less able than light users were to learn word lists; by making a greater number of errors in sorting cards by different characteristics, such as by color or shape; and by making more errors when the rules for sorting the cards were changed without warning. Men in the heavy users group showed somewhat greater impairment than women in the same group.

While the residual cognitive impairments detected in the study were not severe, they could be significant in the day-to-day life of chronic users, Dr. Pope says. The diminished ability to pay attention and decreased mental flexibility exhibited in these tests may cause chronic marijuana users important difficulties in adapting to intellectual and interpersonal tasks, he says.

The residual cognitive impairments
found in the study could be
significant in the day-to-day
life of chronic users.

"This is a fairly definitive study because it was methodologically sound and controlled for a wide number of factors, including the possible confounding effects of alcohol and other drug use," says Dr. Jagjitsing Khalsa of NIDA's Division of Clinical and Services Research.

Previous studies have produced mixed findings about the residual effects of heavy marijuana use on neuropsychological performance, notes Dr. Khalsa. Methodological problems such as ambiguous terminology, failing to take into account cognitive differences in study participants prior to initiation of marijuana use, and failing to note the possible effect of alcohol and other drugs have raised questions about the results of many of these studies. (For more information about methodological issues, see the Pope, Gruber, Yurgelun-Todd article listed below.)

The study by Dr. Pope and Dr. Yurgelun-Todd joins a growing body of well-controlled and well-designed studies that indicate protracted cognitive impairment among heavy marijuana users, says Dr. Khalsa. For example, in 1993, Dr. Robert Block of the University of Iowa College of Medicine compared adult heavy marijuana users and nonusers ranging in age from 18 to 42 years who had been matched on the basis of their intellectual functioning before the onset of drug use. Subjects who used marijuana frequently-7 or more times weekly for at least 2 years-showed deficits in mathematical skills and verbal expression and selective impairments in memory retrieval processes, the study reported. Although the two studies used different neuropsycho-logical tests, "in general, both studies showed some impairments in cognitive abilities among heavy marijuana users," Dr. Block says.

The study joins a growing body
of research that indicates protracted
cognitive impairment among heavy
marijuana users.

It remains unclear whether marijuana's short-term residual cognitive impairments are due either to a residue of the drug that remains in the brain after marijuana's acute effects have dissipated, to a withdrawal effect from abrupt discontinuation of the drug, or to a neurotoxic effect of the drug on brain structure or function. Research has yet to demonstrate conclusively that chronic heavy marijuana use results in cognitive deficits that persist after a prolonged period of abstinence. However, NIDA-supported animal studies do show structural damage to the hippocampus, a structure critical in learning and memory, from the principal psychoactive ingredient in marijuana.

To help answer these and other questions about the long-term effects of marijuana, both Dr. Pope and Dr. Block plan followup studies to examine the effects of chronic heavy marijuana use on cognition and brain function after longer periods of abstinence. Dr. Block proposes to use neuroimaging techniques to look at heavy users' brains while they perform cognitive tasks to determine whether brain function or structure is altered after abstinence from marijuana for a minimum of 30 hours. Dr. Pope is planning a study to determine if cognitive impairments persist in long-time heavy marijuana users for up to 28 days after they have stopped using marijuana.


Block, R.I., and Ghoneim, M.M. Effects of chronic marijuana use on human cognition. Psychopharmacology 110:219-228, 1993.

Pope, H.G., Jr.; Gruber, A.J.; and Yurgelun-Todd, D. The residual neuropsychological effects of cannabis: The current status of research. Drug and Alcohol Dependence 38:25-34, 1995.

Pope, H.G., Jr., and Yurgelun-Todd, D. The residual cognitive effects of heavy marijuana use in college students. JAMA 275(7):521-527, 1996.

From NIDA NOTES, May/June, 1996

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