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MRI scans of nearly 100 people -- including some diagnosed with psychosis -- associated frequent use of high-potency "skunk" marijuana with damage to the corpus callosum, the largest white matter structure in the brain.
"We found that frequent use of high-potency cannabis significantly affects the structure of white matter fibers in the brain, whether you have psychosis or not," said senior researcher Dr. Paola Dazzan, of the Institute of Psychiatry, Psychology & Neuroscience at King's College London.
Further, her team said the damage appears to be dose-dependent. "This reflects a sliding scale where the more cannabis you smoke and the higher the potency, the worse the damage will be," Dazzan said in a college news release.
This is vital information, given that the potency of street marijuana has increased over the last decade, the study authors said.
Reactions to the findings in the United States were mixed, however.
Dr. Scott Krakower, assistant unit chief of psychiatry at Zucker Hillside Hospital in Glen Oaks, N.Y., agreed that the study "supports the idea that using high-potency marijuana can be detrimental."
The upshot? "People should be aware that using this agent isn't benign, and that there are changes to the brain," he said.
But marijuana advocates said the study had limitations.
The damage observed by the researchers appears to be very minor, amounting to a "2 percent change in the corpus callosum" structure, said Mitch Earleywine, chair of NORML, a group that promotes marijuana legalization.
Earleywine, who is also a professor of psychology at the State University of New York, Albany, added that the researchers did not measure the study participants' memory or brain function. "So we have no idea if this had any impact on anything that matters, like memory or impulsivity or depressive symptoms," he said.
For this study, researchers used MRI to examine white matter in the brains of 56 patients who had been diagnosed with psychosis, as well as 43 healthy people from South East London.
The researchers specifically examined the corpus callosum, an area of the brain particularly rich in receptors that respond to THC, the chemical in pot that produces intoxication.
Participants also were asked about their drug use, including the potency of the marijuana they typically use.
Researchers said they were particularly interested in "skunk" marijuana, since previous studies have shown it induces psychotic symptoms, or a break from reality.
"Skunk" is British slang for pot that is highly fragrant and therefore assumed to have higher potency, said Paul Armentano, deputy director of NORML.
The study found that frequent use of high-potency cannabis was linked to significantly higher mean-diffusivity -- a marker of damage in white matter structure -- in the corpus callosum.
However, Earleywine questioned why the researchers didn't look at the hippocampus or areas associated with memory, which are the brain areas that most marijuana studies consider.
The corpus callosum "is a brain area that usually focuses on communication between the hemispheres, or tasks like finding the words for your emotions or having the right hand know what the left hand is doing," he said, adding that the study didn't measure any of these functions.
The researchers also appeared to rely on the participants' own description of both their marijuana use and the potency of their pot, Armentano said.
"It's all rather arbitrary and speculative," he said.
Sean Clarkin is director of strategy and program management with the Partnership for Drug-Free Kids. He said this study should open the door for a healthy "periodic check-in" on a person's marijuana use, to keep it from becoming too frequent or involving increasingly potent pot.
"It provides some scientific basis to say let's be more vigilant than we've been, particularly in the progression from occasional to habitual use," Clarkin said.
The study was published online recently in the journal Psychological Medicine.
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