A recent study by Duke University has found that people repeatedly diagnosed with marijuana dependence during young adulthood had noticeable declines in IQ scores by age 38, especially when the heavy use started in their teens.
The study published online in Proceedings of the National Academy of Sciences, found a small to medium decline in mean IQ between tests taken on the eve of adolescence and again at age 38 was seen in those diagnosed at least three times with marijuana dependence, according to Madeline Meier, PhD, of Duke University, and colleagues, who reported on data from 1,037 participants in a New Zealand birth cohort.
In the small group of participants who became marijuana dependent before age 18 — a total of 23 cohort members — the decline translated to an average of about 8 IQ points, whereas 14 participants who also showed heavy marijuana use but only beginning in adulthood showed only a very small drop in full-scale scores. The researchers also reported that the size of the mean decline increased with the number of marijuana dependence diagnoses that participants had received in five evaluations conducted from ages 18 to 38.
The study findings are consistent with speculation that marijuana use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.
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