A new study suggests that early onset pot use may adversely impact the developing brain. So why are we arresting adults who use it?

By Paul Armentano

Pot prohibitionists are sounding the alarm over a just published study suggesting a decline in IQ among a group of cannabis users who began consuming pot as adolescents and continued persistently for two decades.


An international team of researchers from the US, the United Kingdom, and New Zealand investigated the association between chronic cannabis use and neuropsychological functioning in a group of 1,037 individuals followed from birth. Investigators tested the subjects – all from Dunedin, New Zealand – at age 13, before any participant had ever used cannabis, and again at age 38.

Investigators reported that a small subset of users who began consuming pot prior to age 18 showed declines in IQ at age 38 compared to non-users. The most persistent users showed the greatest levels of impairment in the domains of executive functioning and processing speed.


Among the study’s 38 most seriously “dependent” users, researchers reported a greater than 6 point decline in intelligence quotient. Investigators reported that these differences persisted after they controlled for subjects’ use of other drugs and education – and even after participants ceased consuming cannabis for an extended period of time. By contrast, investigators reported no comparable adverse effects on the brains of those participants who began using cannabis after age 18, including those who continued to use pot long-term.


“In fact, adult-onset cannabis users did not appear to experience IQ decline as a function of persistent cannabis use,” the study’s authors determined. “Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” the investigators concluded.

Predictably, Nora Volkow, director of the prohibitionist-leaning US National Institute on Drug Abuse (NIDA), validated the study’s findings, telling the Associated Press: “I think this is the cleanest study I’ve ever read.” Others disagree, noting the study’s small sample size of “dependent” users and other potentially problematic methodological issues limit the potential significance of study’s findings.


Notably, several previous studies assessing cannabis use and cognition, including its potential impact on IQ have not identified similar, long-lasting changes in performance between users and non-users, and some have even indicated that cannabinoids may be neuroprotective against alcohol toxicity.

Yet, even if one is to accept the study’s findings at face value, it’s hard to see how concerns regarding the potential impact of cannabis on the developing adolescent brain are any way a persuasive argument in support of present day marijuana prohibition. After all, virtually no one wants kids as young as 12 or 13 years of age consuming a mood-altering substance like cannabis. Yet, under cannabis criminalization – a policy that prohibits its use for people of all ages and compels all consumers to acquire the product on the black market instead of from licensed businesses – teens are more likely to have easy access to pot, not less. Need proof? Just ask them.

According to the US government, over 8 out of 10 high-schoolers claim that pot is “fairly easy” or “very easy” to get, a figure that has remained virtually unchanged during the 36 years that the federal government has been asking the question.


Separate national surveys also show that it is now easier for teens to obtain weed than it is for them to acquire booze or tobacco, with at least one recent survey indicating that an estimated 25 percent of teens can now purchase pot in less than an hour – or about as quickly and easily as they could have a pizza delivered to their door.

Predictably, teens now say that they are using less alcohol and cigarettes, but consuming more pot. Specifically, a June 2012 study by the Centers for Disease Control reported that more teens are smoking pot than cigarettes.


Not so coincidentally, teens’ declining use of cigarettes has run parallel to increased state and federal efforts to penalize those licensed businesses that improperly sell to minors and to educate the public about the health risks associated with tobacco. Ditto for booze.


In short, it’s legalization, regulation, and public education – coupled with the imposition and enforcement of appropriate age restrictions – that most effectively keeps mind-altering substances out of the hands of children and reduces the likelihood of their abuse.


Isn’t it about time we took this same approach for pot?

“Increasing efforts should be directed toward delaying the onset of cannabis use by young people,” the authors of this latest study recommend. How does society best go about achieving this goal? It does so by abandoning America’s nearly century-long love affair with cannabis criminalization and replacing it with a pragmatic regulatory framework that embraces the plant’s relegalization.


Paul Armentano is deputy director of NORML.