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Analysis shows cognitive impairment in long-term marijuana users

September 20, 2012
by Gary Enos
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Newly published research findings about heavy marijuana use’s cognitive effects could step up prevention efforts targeting the youngest users. Published online in late August in the Proceedings of the National Academy of Sciences, the research indicates that IQ decline is more pronounced in users who started in their adolescent years as compared with those who initiated use as adults.

The Duke University research analyzes a sample from the Dunedin Study, a prospective study of 1,037 individuals followed from birth to age 38. This analysis is the first long-term study to compare intellectual functioning in young people before their first use of marijuana (with functioning measured at age 13) and then again after 20-plus years of marijuana use.

The study found that the individuals who had more persistent marijuana dependence experienced a steeper decline in IQ and more cognitive impairment, with the latter concern particularly evident in the areas of executive function and processing speed.

According to a statement issued this week by the National Institute on Drug Abuse (NIDA), the findings that the youngest initiators experienced the more significant declines “are especially troubling, given recent data showing increased marijuana use among teens over the last five years, along with declines in perceived risk of harm associated with use.”

The statement added, “The results of this study are consistent with the notion that cannabis may actually cause some of the neuropsychological deficits seen in regular cannabis users.”

The study, funded by the National Institutes of Health (NIH), also reported that cessation of marijuana use did not lead to complete restoration of neuropsychological functioning in those who started smoking in adolescence.