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Link Between Cannabis and IQ Drop Challenged


in Cannabis,Research

A report recently published in the online journal PNAS challenges the belief that smoking cannabis in adolescence can reduce IQ. The original causal link was suggested by the Dunedin longitudinal study of cannabis users in New Zealand (see our original post here). However, new analysis by Norwegian economist Ole Røgeberg indicates that the observed decrease in IQ in this sample could in fact be linked to lower socioeconomic status.

Røgeberg claims that the IQ of children with low socioeconomic status is temporarily boosted by schooling, but when they leave school and become adults their IQs return to the lower baseline. If many of the cannabis users in the Dunedin study were former low socioeconomic status children then this could create the false impression that cannabis was the responsible factor, when actually the small IQ drop is only associated with these individuals leaving school.

Professor Richie Poulton, one of the authors of the Dunedin study, has responded to Røgebergs analysis, maintaining that the original conclusion still stands. He reports that the downward trend in IQ persists even when participants with low childhood socioeconomic status are excluded from the analysis. Furthermore, he states that the IQ scores of participants were the same from the time they began school to adolescence, thus undermining Røgeberg’s claim that schooling temporarily boosts IQ.

However, the statistical power of the data collected in the Dunedin study has been called into question elsewhere, and it is believed that the matter will not be resolved until much larger longitudinal studies and more rigorous statistical analysis of the data have been performed.

Whether or not the link is proved or disproved, the same question should be raised over the use of other substances. Alcohol is one of the most widely abused substances, what threat does it pose to a drinker’s intelligence? Perhaps alcohol is in fact many times worse than cannabis for damaging a persons intellect, but we are unaware as the necessary studies haven’t been performed.

And what if the relative effects of various drugs on IQ are determined? What does it mean for the regulation of that substance? Just because something has potential harms that doesn’t mean that the best way to minimise those harms is to ban it. In this case, banning the harmful (but popular) substance simply opens the doors to an unregulated criminal market and an unrelenting battle to forcefully shut that market down.

Furthermore, an illicit substance cannot be controlled for its dose or purity, leaving users even more exposed to health risks. With cannabis this is a legitimate concern, given that illegal markets tend towards highly potent THC-rich strains. Beckley Foundation research indicates that the active ingredient THC might be the most harmful to cognitive health, and high levels of THC have been linked to increased risk of psychosis. However, there is also evidence that a higher level of the non-psychoactive component cannabidiol (CBD) in the cannabis might exert a protective effect – but most cannabis on the black market contains little to no CBD.

Plenty of alternatives to prohibition exist, as can be seen in the many and varied methods applied to alcohol and tobacco regulation across the world. Restrictions on advertising, age of legal sale, establishments that can sell the drug etc. All are tried-and-tested measures taken to minimise usage of these substances, especially amongst the young. This shows there a lot of room for alternative approaches between the extremes of strict prohibition and all-out legalisation of drugs.

Besides minimising use, the substances themselves can be regulated to reduce harm (for example, low-tar cigarettes). Given the opposing effects of THC and CBD, one evidence-based policy option to reduce harms from cannabis use would be to enforce a minimum CBD content. This would make cannabis safer, protecting users against the elevated health risk brought on by the high-THC cannabis currently dominating the market.

The effect of adolescent cannabis use on adult intelligence still requires further investigation to determine the causal link, if any. But that knowledge is only half the story. The overall question is how to reduce the harms associated with a drug, a question that demands investigation before we can answer it. Naively assuming that prohibition is the best approach should not be our only response.

[PNAS via Australasian Science]



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