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Because of the powerful psychoactive effect of marijuana, many have always assumed that prolonged use can hurt the brain in a similar way to alcohol but while the effects of alcohol on the human brain have been thoroughly documented, the same can’t be said about marijuana. Luckily, we’re seeing a marijuana research revival in the past decade, prompted by the legalization of the recreational and medicinal use of cannabis in more than 10 states in the US.

There are many contradictory findings, but one thing’s for sure: if cannabis poses long-term risks to brain health, these are nowhere near as harmful as those of alcohol — that’s according to the latest findings of researchers at the University of Colorado Boulder (UCB) and the CU Change Lab.

Your brain on pot

The psychoactive compound in marijuana that provokes its notorious high is a so-called cannabinoid called THC, or Tetrahydrocannabinol. However, cannabis contains over 60 cannabinoids, most of which are not psychoactive. For instance, Cannabidiol  (CBD) — another cannabinoid and possibly the 2nd most famous one after THC — is not only nonpsychoactive but actually blocks the high from THC. Other well studied cannabinoids include Cannabigerol (CBG), which has anti-inflammatory effects, Cannabichromene (CBC), which has both anti-inflammatory and anti-depressant effects, and Tetrahydrocannabivarin (THCV), which promotes appetite (the famous munchies).

These cannabinoids act on receptors throughout our brain, keeping neuron firing and affecting dopamine levels, similarly to alcohol, caffeine, and sugar. Because the short-term effects of marijuana can be very intense, there is a valid concern that long-term use can trigger physical changes in the brain that are not beneficial. Alcohol, whose effect on the brain has been very well studied in the last half a century, is known for a fact to damage the brain. Studies on alcoholics found evidence of brain shrinkage, a common indicator of brain damage, which causes learning and memory problems. Can marijuana cause brain shrinkage like alcohol? That’s what UCB researchers set out to find out.

“Particularly with marijuana use, there is still so much that we don’t know about how it impacts the brain,” said Rachel Thayer, a graduate student in clinical psychology at CU Boulder and the lead author of the study, in a statement. “Research is still very limited in terms of whether marijuana use is harmful, or beneficial, to the brain.”

The team examined the brains of over 1,000 participants, both adolescents and adults, using MRIs. They then compared the results to brain images of alcohol users. To spot the differences, the researchers focused on two of the most important components in the brain: gray matter and white matter.

Gray matter, which has a pinkish-grey color in the living brain, contains the cell bodies, dendrites and axon terminals of neurons — this is where all synapses are, where all the heavy lifting takes place. White matter is made of axons connecting different parts of grey matter to each other. Both components are extremely important for cognition and loss of size in either is seen as a sign of brain damage or the brain not working properly.

“When you look at these studies going back years, you see that one study will report that marijuana use is related to a reduction in the volume of the hippocampus. The next study then comes around, and they say that marijuana use is related to changes in the cerebellum or the whatever,” said study co-author Kent Hutchison, a professor of behavioral neuroscience at CU Boulder and co-director of the CU Change Lab.

“The point is that there’s no consistency across all of these studies in terms of the actual brain structures.”

“With alcohol, we’ve known it’s bad for the brain for decades,” said Hutchison. “But for cannabis, we know so little.”

The MRI scans revealed the alcohol use was significantly associated with a reduction in gray matter size and white matter integrity. The longer the exposure to alcohol, the more pronounced the effects. Marijuana use, on the other hand, was not associated with any long-term impact on the amount of gray matter or white matter. This shows that while there may be some unwanted physical changes in the brain because of long-term marijuana use, these consequences are nowhere near as harmful as alcohol. But this is only the beginning. Hutchinson says that despite there are a lot of claims hinting towards the therapeutic effects of marijuana, much more research is required.

“Considering how much is happening in the real world with the legalization movement, we still have a lot of work to do,” he said.

The findings appeared in the journal Addiction. 

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