Does Cannabis Significantly Impair Brain Function?

How long does it take when discussing marijuana use before someone reminds you of the “facts” of cannabis? Your typical layperson (and some scientists) might mention things such as addiction, psychosis, and long-term cognitive dysfunction. While all of these “facts” are shouted from the spire as if they were established mathematical theorems, very little evidence exists which prove these assumptions.

The Daily Mail is one such place where vociferous weed whacking is the norm. They provide plenty of misinformation to those interested in supporting emotional arguments rather than scientific ones. Very often you’ll see articles featuring “reefer toking” brain-addled, hooligans on a robbing spree and whatnot.

As society braces for full legalization, detractors will be pouring from their hives reminding us that cannabis is not a harmless substance that simply makes you giggle and eat cupcakes. (Wait, it’s not?). In fact, they will tell you that marijuana is actually quite dangerous-even as a medicine.

But don’t become alarmed-not yet anyway. The facts speak differently when it comes to preserving brain function in chronic cannabis use.


One such dangerous adverse effect is generating a lot of fog: cognitive dysfunction (CD). Sure sounds scary doesn’t it? What does the term mean when your cognition is defunct? How does it apply to cannabis smokers?

Recently, a meta-analysis (a study of many past studies) by Grant et al.[ref]J Int Neuropsychol Soc. 2003;9:679–89.[/ref] identified residual verbal memory impairments as the most consistent [cognitive dysfunction] deficit associated with chronic cannabis use in otherwise healthy users. Translated that means basically a decrease in short-term memory.

Below are two excerpts from a list of harms from a 2014 NIDA study. I will return to this study in another follow up article. These points cover other features of CD, or “cognitive dysfunction.”

In addition, adults who regularly smoked the substance during adolescence have impaired neural connectivity in areas that involve alertness, awareness, learning, and memory compared with those who have never smoked marijuana.

They also have less functional connectivity in areas that include processing habits and routines and inhibitory control, as well as a significant decline in IQ.

The references used from the above NIDA study was based on previous research which suggested that there were areas of the brain physically altered from cannabis use. These have been debunked by this most recent study:

No statistically significant differences were found between daily users and nonusers on volume or shape in the regions of interest. Effect sizes suggest that the failure to find differences was not due to a lack of statistical power, but rather was due to the lack of even a modest effect. In sum, the results indicate that, when carefully controlling for alcohol use, gender, age, and other variables, there is no association between marijuana use and standard volumetric or shape measurements of subcortical structures[ref] Original: ( 07/13/2015[/ref]

They were looking at physical variations in key subcortical brain structures to see if they were altered from cannabis use. They found no changes.

This study contradicts the facts from the NIDA publication above and renders it obsolete. It was a well-executed trial too. Based on this we conclude that chronic marijuana use does not permanently alter brain structure as previously thought.