Marijuana research has been hampered over the last several decades by the ban on its use and sale, as well as its classification by the DEA as among the most harmful of illegal substances — on par with heroin and LSD. Advocates of legalization argue in part that more medical research can be performed if the DEA downgrades cannabis on its drug schedule so that a clearer picture of marijuana’s true benefits and dangers can be determined. They may not be terribly happy with the latest result in that effort, however. Northwestern University has found that even casual marijuana use is connected with development of brain abnormalities, which could put a dent in the legalization movement:

The days when people thought only heavy Cheech-and-Chong pot smokers suffered cognitive consequences may be over. A study in The Journal of Neuroscience says even casual marijuana smokers showed significant abnormalities in two vital brain regions important in motivation and emotion.

“Some of these people only used marijuana to get high once or twice a week,” said co-author Hans Breiter, quoted in Northwestern University’s Science Newsline. Breiter hailed the study as the first to analyze the effects of light marijuana use. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school,” he said. “Our data directly says this is not the case.”

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” he added.

There’s a big problem with this study, though, and it relates directly to the complaints about the inability to properly study marijuana. The sample size is less than impressive, with just 20 each in the study and control groups:

For their most recent study, Breiter and his team analyzed a very small sample of patients between the ages of 18 and 25: 20 marijuana users and 20 well-matched control subjects.  The marijuana users had a wide range of usage routines, with some using the drug just once or twice a week and others using it every single day.

Utilizing magnetic resonance imaging (MRI), the researchers analyzed the participants’ brains, focusing on the nucleus accumbens (NAC) and the amygdala – two key brain regions responsible for processing emotions, making decisions and motivation.  They looked at these brain structures in three different ways, measuring their density, volume and shape.

According to Breiter, all three were abnormal in the casual marijuana users.

“For the NAC, all three measures were abnormal, and they were abnormal in a dose-dependent way, meaning the changes were greater with the amount of marijuana used,” Breiter said.  “The amygdala had abnormalities for shape and density, and only volume correlated with use.  But if you looked at all three types of measures, it showed the relationships between them were quite abnormal in the marijuana users, compared to the normal controls.”

Very small indeed. This looks more like a pilot study to generate more funding than a study on which public policy should be based. The small size is a little mystifying, too; surely Northwestern could have found more than 20 students and recent graduates who are at least casual users of marijuana. (Or perhaps they had a tough time finding more than 20 who didn’t.)

That said, the results seem significant enough that casual marijuana users might want to rethink their choices. Northwestern has enough academic detachment from the issue to make its work stand out. And if further studies duplicate this outcome on a larger scale, it will put a very big dent in the “pot is less harmful than alcohol” argument that has helped broaden the legalization coalition.