Of course, correlation is not the same as causation, and there are still reasons to be skeptical of the findings and practical significance:
- The sample size was small: 46 people in each group. Thus, very sensitive to potential selection bias, reporting bias, and confounding. (Apparently from a larger brain study of over 2000 people, they could only find 47 individuals who had only tried cannabis once or twice by that age, and one had an inconclusive scan.)
- Another way that size matters is "effect size". And while the numbers are difficult to interpret, they are also on the small side.
- Some underreporting of cannabis (and other substance) use is always very likely, and can skew results in either direction depending on what is being underreported and by whom.
- Temporality of the initial findings remains unclear (i.e. which occurred first?).
- No other study has ever found long-lasting brain changes from a single dose or two of cannabis, at least not in humans. And no one has ever replicated these findings yet.
- Previous studies have been quite inconsistent in terms of the effects (or lack thereof) of cannabis use, even regular use, on brain structure, function, and cognition. Some studies found increases in gray matter, others decreases, still others null. Ditto for white matter as well.
- Given how the participants were European, it is particularly difficult to disentangle cannabis and tobacco use as the two substances are customarily mixed. This is important because nicotine is a known neurotoxin to which the early adolescent brain appears to be exquisitely sensitive.
- The study apparently did not look at the brain effects of those who used cannabis but did not initiate until age 15 or older, so even if the effects are real, they may not necessarily be generalizable to people over 15.
- The practical significance of the findings is clear as mud right now. Even the researchers aren't entirely sure what they mean. Is it actually bad to have more gray matter, and compared to what exactly? Is it disrupted brain development, or simply stimulated and enhanced neurogenesis?
- While greater gray matter in the study was correlated with differences in psychomotor performance contemporaneously for whatever reason, the only bad thing correlated with it two years later was generalized anxiety. No other measures of psychopathology or performance were correlated at baseline or two years later.
- Come to think of it, if these 14 year olds were followed up two years later, why were their brains not scanned again a second time at 16, especially since there admittedly were many who were cannabis-naive at 14 who went on to use it two years later?
- And frankly, it really doesn't even pass the straight face test that a mere single dose or two of such a relatively mild psychoactive substance would be enough cause a long-lasting disruption to brain development, a process that occurs over several years. To quote Paracelsus, "the dose makes the poison". That, and Occam's Razor too.
Of course, the cliche that "more research is needed" certainly applies here, and the researchers indeed say as much. They also plan to do a follow-up study as well in the near future. Though if history is any indication, we should not hold our breath waiting for these results to be replicated.
Keep in mind that the infamous 2012 study that reportedly found persistently reduced IQs among adults who used cannabis before age 18, was debunked by 2014 study that found no correlation between adolescent cannabis use and IQ or exam performance (though heavy use beginning before age 15 was associated with slightly poorer exam results at age 16). This latter study did control for tobacco, alcohol, and a host of other factors. So it is very likely that soon another study will come a long and refute the first study discussed in this article, or perhaps find that any such effects are limited to the heaviest users, particularly those who began before age 15 or 16. In fact, a 2018 systematic review of 69 studies of adolescent and young adult cannabis use and cognitive functioning found that reported adverse effects were much smaller in size than the prohibitionists like to claim, and generally tend to be temporary rather than permanent, even for frequent and/or heavy use. And interestingly, no correlation with age of onset, though the mean age of study participants in these 69 studies was significantly higher than in the aforementioned Montreal study.
Other studies as well cast serious doubt on the scary claims of cannabis neurotoxicity as well, and most studies find weed safer than alcohol.
So what is the best takeaway from such studies? It would seem that while occasional or moderate cannabis use is basically a non-problem, heavy and/or daily/near-daily use (unless medically necessary) should probably be avoided at any age, but particularly for people under 18 and especially under 15. And while delaying the onset of use, or at least regular use, for as long as possible is probably wise for people under 18 and especially under 15, there is no hard scientific evidence that cannabis is any more harmful at 18 than it is as 21, 25, or even 30 for that matter. Thus, there is no good reason to keep it illegal or set the age limit any higher than 18. And even for people well under 18, the criminal law is still far too harsh a tool to apply to something like this that more likely than not turns out to be a non-problem.
The prohibitionist "scientists" and their pal-reviewers really seem to be grasping at straws now. Apparently they think if they move the goalposts enough, they will score a touchdown. But as we all know, that is not how science really works.
