Studies Show Long-Term Effects of Cannabis on the Brain

Cannabis has proven to be both damaging and addictive in two recent studies.

Posted Apr 29, 2014

Flickr/HeyPaulStudios/CC License
Cannabis damages the brain.
Source: Flickr/HeyPaulStudios/CC License

With the legalization of recreational marijuana in Washington and Colorado, questions about cannabis have vaulted into the public consciousness. For a couple of minutes, let’s put aside policy concerns and look just at the effects of cannabis on the brain. Two studies published in top journals, both in April 2014, look inside human brains at the long-term effects of cannabis use.

Is cannabis addictive? Is it safe? Let’s consider the evidence.

First, an article in the Journal of Neuroscience used MRI scans to look inside the brains of young, recreational marijuana users at regions associated with addiction. Previous studies have shown that other drugs known to be addictive affect the brain’s reward centers—especially the brain’s amygdala, which controls emotional learning, and a structure called the nucleus accumbens, which controls pleasure (including our ability to laugh). We’ve also known that adding cannabis-based chemicals to the brains of rats creates changes in these structures related to addiction. But it’s quite a leap from introducing cannabinoids to rat brains and knowing the effects of smoking pot on humans.

So a team of Harvard-led researchers recruited 40 young adults—20 marijuana users and 20 non-users—to see if what is true in the brains of rats is also true in the brains of college students. Sure enough, human marijuana users had changes in the volume, density, and topography in both the amygdala and nucleus accumbens.

“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,” the researchers write.

This study, and the studies that led up to it, show that marijuana use creates physical changes in the brain associated with addiction. And, the researchers point out, these results were seen in non-dependent, young adult users. What are the effects of even heavier pot use on the brain?

That’s the question of the second study, published in the Nature journal, Neuropsychopharmacology. Again, the study used MRI imaging to ask if the effects of cannabis-based chemicals seen in rat brains are also seen in human brains. This time, the study compared heavy marijuana smokers to occasional smokers to see if overall brain changes are more extreme, the more you smoke. And it looked outside just the structures of addiction to explore changes in overall brain structures: How does marijuana use affect the brain?

The study found reduced grey matter volume in nearly all brain regions that are rich in the “receptors” that can trap and respond to cannabis-based chemicals. These regions include a long list of structures, almost all of which are part of a network that controls motivation, emotion, and emotional learning.

Here’s an important part: The degree to which these brain areas changed was due to one of two things—either heavy use or starting use during adolescence. Long-term, heavy users had the same reductions in grey matter volume as lighter users who started in their teens.

So let’s revisit our two questions. Is marijuana addictive? Yes, and there's a real, visible change in the brain’s reward system. And is marijuana safe? No, and the younger you start, or the more you use over time, the more dangerous it is to your brain.

Whether or not you believe recreational marijuana should be legal, it’s time to admit its power as a dangerous, addictive drug.