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February 21, 1996

The Residual Cognitive Effects of Heavy Marijuana Use in College Students

Author Affiliations

From the Biological Psychiatry Laboratory (Dr Pope) and the Brain Imaging Center (Dr Yurgelun-Todd), McLean Hospital, Belmont, Mass, and Department of Psychiatry, Harvard Medical School, Boston, Mass (Drs Pope and Yurgelun-Todd).

JAMA. 1996;275(7):521-527. doi:10.1001/jama.1996.03530310027028

Objective.  —To assess whether frequent marijuana use is associated with residual neuropsychological effects.

Design.  —Single-blind comparison of regular users vs infrequent users of marijuana.

Participants.  —Two samples of college undergraduates: 65 heavy users, who had smoked marijuana a median of 29 days in the past 30 days (range, 22 to 30 days) and who also displayed cannabinoids in their urine, and 64 light users, who had smoked a median of 1 day in the last 30 days (range, 0 to 9 days) and who displayed no urinary cannabinoids.

Intervention.  —Subjects arrived at 2 PM on day 1 of their study visit, then remained at our center overnight under supervision. Neuropsychological tests were administered to all subjects starting at 9 AM on day 2. Thus, all subjects were abstinent from marijuana and other drugs for a minimum of 19 hours before testing.

Main Outcome Measures.  —Subjects received a battery of standard neuropsychological tests to assess general intellectual functioning, abstraction ability, sustained attention, verbal fluency, and ability to learn and recall new verbal and visuospatial information.

Results.  —Heavy users displayed significantly greater impairment than light users on attentional/executive functions, as evidenced particularly by greater perseverations on card sorting and reduced learning of word lists. These differences remained after controlling for potential confounding variables, such as estimated levels of premorbid cognitive functioning, and for use of alcohol and other substances in the two groups.

Conclusions.  —Heavy marijuana use is associated with residual neuropsychological effects even after a day of supervised abstinence from the drug. However, the question remains open as to whether this impairment is due to a residue of drug in the brain, a withdrawal effect from the drug, or a frank neurotoxic effect of the drug.(JAMA. 1996;275:521-527)

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