Early cannabis use damages developing brain

Aug 30 2012

By Helen Albert, medwireNews Reporter

Persistent use of cannabis before the age of 18 years can lead to lowered intelligence, attention, and memory in adulthood, show results published in the Proceedings of the National Academy of Sciences.

The researchers believe that the neuropsychologic declines observed in these individuals are likely to be due to damage caused to the developing adolescent brain by overexposure to the drug, especially as giving up cannabis only resulted in partial restoration of function.

"Marijuana is not harmless, particularly for adolescents," said study author Madeline Meier (Duke University, Durham, North Carolina, USA) in a press statement.

Meier and team used data from the Dunedin Study birth cohort in New Zealand to assess the effects of cannabis use.

In total, 1037 individuals were followed up from birth to the age of 38 years. Cannabis use and level of dependence was evaluated at age 18, 21, 26, 32, and 38 years using the Diagnostic Interview Schedule and Diagnostic and Statistical Manual of Mental Disorders Criteria. Various neuropsychologic tests were carried out at age 13 years (before any cannabis use) and at age 38 years, including those assessing IQ and memory.

The team found that persistent cannabis use or dependence before the age of 18 years (at least once a week) resulted in broad neuropsychologic declines across different domains of function, even after controlling for years of education and use of alcohol or other drugs.

For example, persistent users had an average decline in IQ of 8 points between the age of 13 and 38 years, which could result in a drop from the 50th to the 29th percentile of the population if a person starts with an IQ of 100. Cognitive problems were also more common for persistent cannabis users.

"Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come," Meier commented.

Notably, the impairments observed by the researchers were restricted to those who persistently used cannabis in adolescence and the authors say this suggests a "neurotoxic effect of cannabis on the adolescent brain."

They add that their findings "highlight the importance of prevention and policy efforts targeting adolescents."

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