It seems like something straightforward in essence, but apparently being a heavy cannabis smoker as a teenager results in a significant cognitive decline in adulthood, something that is not seen comparably in adult users of the drug.
Clinical psychologist Madeline Meier at Duke University in Durham, North Carolina and her colleagues published their findings in the journal Proceedings of the National Academy of Sciences. The research group used data from the Dunedin Longitudinal Study, which is an ongoing multi-factor survey involving 1,037 New Zealanders that were followed from birth. Currently, the survey has about 40 years’ worth of data, and participants in the study have been periodically tested for IQ and other neuropsychological indices over the years.
When the researchers tested the subjects’ adult IQ at 38 years old, the heaviest and most persistent adolescent-onset users in this study had experienced an average decline of 8 IQ points from childhood to adulthood. Non-users increased their IQ during this period by around one point. There still was a decline of some IQ points in non-heavy cannabis users. The drop in mental acuity seemed irreversible, even after users had quit marijuana. Persistent users who started smoking as adults didn’t seem to experience this IQ decline.
The study allowed the researchers to dismiss the effect of education by finding no difference in the proportion of heavy cannabis users in the study and those with only a high-school education or less. They could also control for complicating factors, such as schizophrenia, the use of other drugs, or the fact that subjects might have been under the influence of cannabis during testing.
There are neurotoxic effects from cannabis at a critical age for brain development, which wasn’t isolated in this particular study. The underlying mechanism is still somewhat of a mystery.
The data is based upon a heavy cannabis user sample of just over 50 people, so it’s hard to draw certainties from it. Also, the effect of the active ingredient in cannabis, tetrahydrocannabinol (THC) needs to be explored, as there has been a significant increase of its amount in marijuana. Back in the 1980s, when the Dunedin Study subjects were kids, the content of THC in cannabis was between 4-5%. Today, cannabis has 16-18% of THC, so the effects of mental decline are most likely magnified.
Reference: “Persistent cannabis users show neuropsychological decline from childhood to midlife” by Madeline H. Meier, Avshalom Caspi, Antony Ambler, HonaLee Harrington, Renate Houts, Richard S. E. Keefe, Kay McDonald, Aimee Ward, Richie Poulton and Terrie E. Moffitt, 27 August 2012, Proceedings of the National Academy of Sciences.