High-potency pot smokers show brain-fiber damage

More THC associated with weakness in white matter linking cerebral hemispheres


SKUNKED  A new study links marijuana with high levels of THC to damage in a major brain connector. 


People who use especially potent pot show signs of damage in a key part of their brain. The results, reported online November 27 in Psychological Medicine, are limited, though: The small brain scanning study doesn’t show that marijuana caused the brain abnormality — only that the two go hand-in-hand. But the findings suggest that potency matters, says study coauthor Tiago Reis Marques, a psychiatrist at King’s College London.

“We are no longer talking about smoking cannabis or not smoking cannabis,” Reis Marques says. Just as vodka packs more of a punch than beer, a high-potency toke delivers much more of the psychoactive substance tetrahydrocannabinol, or THC. A bigger dose of THC may have stronger effects on the brain, Reis Marques says.

That’s important because as marijuana plant breeders perfect their products, THC levels have soared. Samples sold in Colorado, for instance, now have about three times as much THC as plants grown 30 years ago, a recent survey found (SN Online: 3/24/15).

Reis Marques and his colleagues scanned the brains of 43 healthy people, about half of whom use cannabis. The researchers used a method called diffusion tensor imaging to study the structure of the brain’s white matter, neural highways that carry messages between brain areas. Participants gave a detailed history of their past drug use, including information about how potent their marijuana was.

POT HEAD The corpus callosum — white matter that links the left brain to the right — is weaker in people who smoke high-potency cannabis, a new study suggests. TAbildskov/Wikimedia Commons (CC BY-SA 3.0)

People who reported using high-potency marijuana showed signs of damage in the corpus callosum, the major white matter tract that connects the left side of the brain to the right. Water molecules diffused more easily along tracts of the corpus callosum, a sign that the tissue is weaker.

While the results show a link between smoking high-potency cannabis and white matter damage, they can’t prove that cannabis caused the trouble. “These people could have had deviant brain structures prior to use — they weren’t randomly assigned,” says psychologist Mitch Earleywine of the University at Albany in New York. The results could be explained by other drug use, too, he says. Cocaine, for instance, has been tied to corpus callosum abnormalities, says Earleywine, who serves on the advisory board of NORML, the National Organization for the Reform of Marijuana Laws.

Because the experiment focused only on brain anatomy, it’s unclear whether these changes would affect abilities like memory, impulsivity or depression. It’s also unknown whether white matter tracts elsewhere in the brain are affected by THC content, says neuroscientist and psychiatrist Hans Breiter of Northwestern University in Evanston, Ill. “This study leaves out what is occurring with the rest of the white matter,” he says. It will be important to look at other tracts, particularly those involved with memory and other behaviors that marijuana might influence, he says.

With the growing availability of supercharged marijuana, understanding exactly what it does to the brain is more important than ever, Reis Marques says, particularly for young people who may not realize the marijuana they are using is much more powerful than earlier versions. “We are in a stage where there is missing information, or the information is changing fast,” he says. 

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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