The existing literature on the long-term effects of marijuana on the brain is confusing, mostly due to methodological differences across studies. However, in a new study of cannabis users, scientists investigated the drug’s long term impact through brain scans, hoping to overcome methodological problems in previous studies. They found that marijuana use changes the brain.
The team studied 48 adult cannabis users aged 20 to 36 and compared data with a group of matched non-users. Researchers collected multimodal measures of chronic marijuana using adults with a wide age range that allows for characterization of changes across lifespan without developmental or maturational biases as in other studies.
Compared with controls, marijuana users had significantly less bilateral orbitofrontal gyri volume, higher functional connectivity in the orbitofrontal cortex (OFC) network, and higher structural connectivity in tracts that innervate the OFC (forceps minor) as measured by fractional anisotropy (FA). Increased OFC functional connectivity in marijuana users was associated with earlier age of onset. Lastly, a quadratic trend was observed suggesting that the FA of the forceps minor tract initially increased following regular marijuana use but decreased with protracted regular use.
The findings suggest that chronic marijuana use is associated with complex neuro-adaptive processes and that onset and duration of use have unique effects on these processes. In other words, if you start using marijuana at a young age and use it often, it affects the way your brain works, and not for the good.
Regular cannabis use shrinks the brain, but increases the complexity of its wiring. The loss of brain volume is balanced to some extent by more connections between neurons. This seems to be the brain’s way of trying to compensate for the changes made by marijuana use. Tests showed that regular users also had lower IQs than non-users and this did not appear related to abnormalities of the brain.
Brain scans disclosed that smoking cannabis every day was associated with shrinkage in the region of the brain involved in mental processing and decision making. Marijuana smokers who started taking the drug at a young age showed greater structural and functional connections between their brain neurons, the research showed. After six to eight years of continually smoking cannabis, the increases in structural wiring in the brain declined, but users continued to display higher connectivity than non-users. Again, this seems to show that the brain is trying to compensate for the damage being done to it by the marijuana use.
Dr Sina Aslan, from the University of Texas, Dallas, who co-led the research, said:
“The results suggest increases in connectivity … that may be compensating for grey matter losses. Eventually, however, the structural connectivity or ‘wiring’ of the brain starts degrading with prolonged marijuana use.”
Although the study does not conclusively address whether any or all of the brain changes are a direct consequence of marijuana use, the long-term effects on brain structure do suggest that these changes are related to age of onset and duration of use. The younger you start using marijuana and the more often you use it, the worse off your brain will be.
Further work is needed to determine whether stopping cannabis use reverses the changes and if occasional users suffer similar effects. The best advice is not to use marijuana until your brain is fully developed, around the age of 25. The changes in the brain caused by early and prolonged marijuana use are serious and not worth the short term pleasure of the high.