Much has been made of this recent study (probably behind a paywall unless accessed via a university network, sorry) out of Australia that purports to show that adolescent cannabis usage leads to "serious brain damage".
Actually, the paper does not purport to show such a thing at all; the reporting on the paper and the occasional triumphalist conservative blogger do.
What the paper does show is a significant alteration in the connectivity of certain white matter tracts correlating to earlier onset of cannabis use in a fairly small (59 people) cohort of users compared with a smaller (33) group of non-users. These "connectivity maps" are based on an algorithm that is rather impenetrable even for a pretty well-educated non-expert in diffusion-weighted MR imaging (i.e., me), so I won't get into it...suffice it to say that they looked at the diffusion of water through white matter structures and made inferences about their microstructure--specifically, how many interconnections existed among the axons (think of them as the "cables" of the brain, if you like).
Here is what the paper does not show:
1. Any evidence of measurable cognitive impairment in their cohort (they did not test this.)
2. Any evidence of increased psychiatric disorders in their cohort, other than to say: "Cannabis users had significantly greater trait anxiety and depressive
symptoms, and smoked significantly greater amounts
of tobacco than non-users. However, no participant
had ever been diagnosed with an anxiety or depressive disorder or had
treatment for such symptoms." [emphasis added] In other words, those traits were sub-clinical. It is every bit as possible (if not likely) that people with sub-clinical anxiety and/or depression are more likely to be cannabis users, rather than the other way around.
Oh, and there are several studies that have correlated tobacco use with white matter abnormalities.
The authors did do a post hoc analysis of their data and found that the correlation between cannibus use and decreased axonal connectivity "remained significant" when taking these possible confounding variables into account. All well and good, but the fact that they do not show the actual results of this post hoc analysis (i.e., we don't really know how much of an impact taking those variables into account had) is possibly telling. I'm kind of surprised the reviewers let them get away with that.
3. Most importantly: the relationship between axonal connectivity and any particular functional deficit is...more or less completely unknown. The authors write: "Disturbed
brain connectivity in cannabis users may underlie
cognitive impairment and vulnerability to psychosis, depression and
disorders (Lim et al., 2002), all of which are significant public health concerns." Trust me, because I've written no small number of these things: this is science-speak for "we think that A leads to B, but nobody's proven it yet and we don't here, either, and there is potential public health relevance so please continue to fund our research."
I want to emphasize here that I think the researchers have made an important contribution, and that I do not think that they've done anything in bad faith, except possibly fall for the temptation that far too many in our profession do of talking up their results to the press. And there may very well be clinically important effects here; that certainly merits further investigation. However, these data (as always) must be taken in the context of every other data set addressing the same general question, which--as the authors themselves point out in the very first sentence--are "equivocal".
As to how or whether this ought to inform policy vis a vis the legal status of marijuana: no one is seriously lobbying for it to be legal for adolescents to use.
And of course, parents have much more compelling reasons to steer their children away from marijuana (and drug use generally, including alcohol) in adolescence than any possible developmental effects. Those are the years you should be doing your homework, learning new skills, learning how to interact socially, and generally learning how to be a responsible decent human being...which includes developing the skill set necessary (if you are so inclined) to indulge in the occasional intoxicant while remaining a productive member of society...as an adult.