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March 6, 2002

Cannabis, Cognition, and Residual Confounding

Author Affiliations

Author Affiliation: Biological Psychiatry Laboratory, McLean Hospital, Harvard Medical School, Belmont, Mass.

JAMA. 2002;287(9):1172-1174. doi:10.1001/jama.287.9.1172

In this issue of THE JOURNAL, Solowij and colleagues1 report a variety of neuropsychological deficits in long-term cannabis users who were tested a median of 17 hours after their last reported cannabis intake. Their findings of impairments in memory and attention are not surprising since several large and well-controlled studies have found similar deficits on neuropsychological tests administered to long-term cannabis users after 12 to 72 hours of abstinence.25 If these deficits are brief and reversible (ie, due to a residue of cannabinoids lingering in the brain or to withdrawal effects from abruptly stopping the drug), they might not be a serious threat. However, if these deficits are prolonged or irreversible (ie, due to neurotoxicity from years of cumulative cannabis exposure), they become a matter of grave concern. The findings of Solowij and colleagues favor the latter possibility in that longer-term cannabis users in the study often showed significantly greater deficits than shorter-term users, and neuropsychological performance measures were often negatively correlated with lifetime duration of use. Furthermore, these correlations could not be explained by greater withdrawal symptoms or heavier recent cannabis consumption among the longer-term users. Solowij and colleagues1 conclude that "our results confirm that cognitive impairments develop as a result of prolonged cannabis use . . . and [that] they worsen with increasing years of use."

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