By O’Shaughnessy’s News Service 8/28/12

“Heavy Pot Use Tied to IQ Drop,” is how MedPage Today played  a study published  online in Proceedings of the National Academy of Sciences. We expect many more headlines to blare out the conclusions of the research team led by Madeline Meier, PhD, of Duke University.  MedPage Today provides two “action points” at the top of their coverage for busy healthcare providers who prefer reading summaries of summaries:

• Individuals repeatedly diagnosed with cannabis dependence during young adulthood had noticeable declines in IQ scores by age 38, especially when the heavy use started in their teens.

• Note that the study findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.

The study by Meier et al involved a grand total of 23 individuals who had been diagnosed as “cannabis dependent” at least three times before age 18 (!) 

The researchers compared scores from IQ tests taken before adolescence (ages 7 through 13) and again at age 38. They found that those who used heavily before age 18 lost an average of about eight IQ points by age 38, whereas those whose heavy use began after 18 lost almost zip.

The heavy users had been identified by the Dunedin Multidisciplinary Health and Development Study, which has tracked 1, 037  individuals born between April 1972 and March 1973 in a southern New Zealand City.  Everyone in the Dunedin cohort gets interviewed every five years. In this process, 242 never reported cannabis use; 479 reported use but were not diagnosed with “dependence;” 80 were diagnosed with dependence once, 35 twice, and 38 three or more times.

“Adult-onset cannabis users did not appear to experience IQ decline as a function of persistent cannabis use,” according to the investigtors.  They describe their findings as “consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.”

Meier et al are confident their findings about heavy users who start early are valid, and expect the Corporate State Propaganda Machine to warn the masses. “Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence,” they advise.

Duke’s  press release was unrestrained: “Adolescent pot use leaves lasting mental deficits,” said the hed. “The persistent, dependent use of marijuana before age 18 has been shown to cause lasting harm to a person’s intelligence, attention and memory” said the lede.

We are of the opinion that IQ tests are a big shuck. Maybe by age 38 the early, heavy users could no longer take the test questions seriously.  Maybe with every joint they had gotten smarter. More cynical, anyway.

—Fred Gardner, Harvard ’63