In the Journal of Neuroscience a team of scientists led by Jodi Gilman released the results of a brain-scanning study that looked at how the brain structures of casual pot smokers and those who don't smoke differ. With a small sample of people—some college students who smoked pot semi-regularly and others who abstained—the scientists showed that pot smokers' brains are different: some parts of the brain were shaped differently, and pot smokers had more grey matter in others.
Now, knowing the way that the study was done, with the researchers taking a snapshot of the participants' brains with an MRI, gives us two different ways to interpret these results: the right way and the wrong way.
The wrong way, and the approach taken by most news outlets, was to argue that, because the pot smokers' brains were different, it was the marijuana that caused the changes.
That sort of interpretation really isn't appropriate, given the study, says John Gever for MedPage Today, a medical news site.
[T]hese findings only reflected differences between the marijuana users and controls at a single point in time. The researchers did not, could not, demonstrate that the differences resulted from marijuana smoking or even that the "abnormalities" relative to controls reflected changes from some earlier state.
The right way, and indeed the only thing the study can really say, is to say that pot smokers' brains are different in some consistent ways from the control population. Whether the pot made them that way, or whether it was just a coincidence, we can't say.
This is a common problem in researching the effects of drug use. NPR says it's a “chicken-and-egg” dilemma: do pot smokers' brains change, or do the brain differences predispose someone to taking up the drug? Other factors often tied up with drug use, such as socioeconomic factors, can also affect the brain's structure.
With newly lax laws in Washington and Colorado, some researchers think we're irresponsibly rushing into supporting recreational reefer. But disentangling all of the caveats in the research is important, and it's only after this is done that scientists can really blame the substance for any ill effects.