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.

In the study, researchers at Boston's Massachusetts General Hospital and Northwestern University in Chicago performed MRI brain scans on 20 young adult "casual" marijuana users and 20 age- and sex-matched nonusers. They found that, in the users, gray matter densities in the nucleus accumbens were higher than in controls, and the right amygdala and left nucleus accumbens were shaped differently.

Interesting, but remember that these 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.

You wouldn't know that from the media coverage. Here's a sampling of headlines:

Marijuana News: Casual Pot Use Impacts Brains of Young Adults, Researchers Find (The Oregonian)

Study Finds Brain Changes in Young Marijuana Users (Boston Globe)

Casual Marijuana Use Linked to Brain Changes (USA Today)

Even Casually Smoking Marijuana Can Change Your Brain, Study Says (Washington Post)

Study Finds Changes in Pot Smokers' Brains (Denver Post)

Recreational Pot Use Harmful to Young People's Brains (TIME)

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.

To their credit, the study team's actual paper stuck fairly close to their data, concluding that the users showed "structural abnormalities." They only strayed into overinterpretation when they wrote that "the left nucleus accumbens was consistently affected by cannabis use." Nope, it was still just an association, no cause-and-effect shown -- as Breiter and colleagues acknowledged later in their paper.

It might also have been a stretch to call their subjects "casual users." The mean intakes were 3.83 days of use and 11.2 joints per week, and 1.8 smoking occasions per day of use. To me -- and I lived in Ann Arbor in the 1970s -- this sounds more like the profile of a just-short-of-heavy regular user.

I don't want to minimize the paper's genuine importance. The differences in brain structure from controls could well have functional consequences, and could well reflect the effects of marijuana use. Certainly these findings deserve follow-up.

But until we get it, everyone, please dial back the Reefer Madness hype.

Striking a Nerve is a blog by John Gever for readers interested in neurology and psychiatry.