So, of course, everyone in the media has been gushing about the study that proves that even casual pot smoking damages the brain. Of course, that’s not even close to what the study showed, even if you accept the study itself as legitimate.
The people who really know, know better.
Here’s John Gever, Deputy Managing Editor, MedPage Today: Striking a Nerve: Bungling the Cannabis Story
Correlation does not equal causation, and a single exam cannot show a trend over time. Basic stuff, right?
But judging by coverage of a study just out in the Journal of Neuroscience, these are apparently foreign concepts for many folks in the media. [...]
Sad to say, the Society for Neuroscience (SfN), which publishes the Journal of Neuroscience, may have driven these dramatic overinterpretations by promoting the study in a press release headlined “Brain changes are associated with casual marijuana use in young adults.”
Also note that the study did not identify any cognitive or behavioral abnormalities in the cannabis users versus controls — it was strictly an MRI study.
That, however, didn’t stop senior author Hans Breiter, MD, of Northwestern from opining in the SfN press release that the study “raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences.”
Um, no, it doesn’t — not without before-and-after MRI scans showing brain structure changes in users that differ from nonusers and documentation of functional impairments associated with those changes.
Lior Pachter, a computational biologist, was even harsher about the actual science: Does researching casual marijuana use cause brain abnormalities?
This is quite possibly the worst paper I’ve read all year (as some of my previous blog posts show I am saying something with this statement). [...]
First of all, the study has a very small sample size, with only 20 “cases” (marijuana users), a fact that is important to keep in mind in what follows. The title uses the term “recreational users” to describe them, and in the press release accompanying the article Breiter says that “Some of these people only used marijuana to get high once or twice a week. People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.” In fact, the majority of users in the study were smoking more than 10 joints per week. There is even a person in the study smoking more than 30 joints per week (as disclosed above, I’m not an expert on this stuff but if 30 joints per week is “recreation” then it seems to me that person is having a lot of fun). More importantly, Breiter’s statement in the press release is a lie. There is no evidence in the paper whatsoever, not even a tiny shred, that the users who were getting high once or twice a week were having any problems.
Pachter then gets into an analysis of the study’s bad math (which is completely out of my knowledge base and totally over my head, so I can’t really comment on it, but it sounds damning.
This issue is one of the oldest in the book. There is even a wikipedia entry about it. Correlation does not imply causation. Yet despite the fact the every result in the paper is directed at testing for association, in the last sentence of the abstract they say “These data suggest that marijuana exposure, even in young recreational users, is associated with exposure-dependent alterations of the neural matrix of core reward structures and is consistent with animal studies of changes in dendritic arborization.” At a minimum, such a result would require doing a longitudinal study. Breiter takes this language to an extreme in the press release accompanying the article. I repeat the statement he made that I quoted above where I boldface the causal claim: “”Some of these people only used marijuana to get high once or twice a week. People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.” I believe that scientists should be sanctioned for making public statements that directly contradict the content of their papers, as appears to be the case here.