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Study Finds Link Between Early Pot Use and Lasting Cognitive Deficits - But Is Pot Itself the Culprit?

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April 01, 2003

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Belmont, MA - A new study in the latest issue of Drug and Alcohol Dependence (abstract) indicates an association between early use of cannabis (marijuana) and persisting cognitive deficits.

Led by Harrison Pope, Jr., MD, director of the Biological Psychiatry Laboratory at McLean Hospital, the researchers, from McLean and the National Institute on Drug Abuse (NIDA), compared 122 heavy users of cannabis with 87 minimally exposed control subjects. The 122 heavy users had smoked a median of about 15,000 separate times in their lives; the control subjects had smoked a median of only 10 times.

Among the 122 cannabis users, 69 began smoking the drug before age 17 (early-onset group) and 53 began after age 17 (late-onset group). At the time of the neuropsychological assessments, all cannabis use had stopped for at least 28 days.

The authors found that early-onset cannabis users exhibited poorer cognitive performance than late-onset users and control subjects. Statistical analyses, which adjusted for age, gender, ethnicity and family variables, indicated that early-onset users differed significantly from late-onset users and control subjects on several measures of cognitive function, most notably in verbal IQ. After adjusting for verbal IQ, however, virtually all of the differences between the early-onset users and the late-onset users disappeared. These results suggest that a combination of social and/or biological factors may lead to an association between early use of cannabis and deficits in verbal cognition.

The authors offer three competing explanations for these findings: 1) Cannabis might have a toxic effect on the developing brain of young users; 2) People who begin smoking cannabis at a young age may have lower cognitive abilities initially, before they ever tried cannabis; or 3) Young-onset cannabis users may not learn the cognitive skills required for the tests, as they acquire less education and have less familiarity with mainstream culture.

"Any single explanation, or combination of the three, might fit the facts of the study," said Pope.

"Our results show how difficult it is to disentangle the causes of cannabis-associated cognitive deficits."

Co-authors of the study include Amanda Gruber, MD, James I. Hudson, MD, ScD, Geoffrey Cohane, BA and Deborah Yurgelun-Todd, PhD, from the Biological Psychiatry Laboratory and Cognitive Neuroimaging Laboratory of McLean Hospital, and Marilyn Heustis, PhD, from the Intramural Research Program at NIDA.

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