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Brain abnormalities seen in heavy pot smokers

June 2, 2008
Courtesy Archives Journals
and World Science staff

Two im­por­tant brain struc­tures ap­pear smaller than av­er­age in men who are long-time, heavy pot smok­ers, a study has found.

The find­ings chal­lenge a pop­u­lar view that ma­ri­jua­na is harm­less, wrote the au­thors of the stu­dy, pub­lished in the June is­sue of the re­search jour­nal Ar­chives of Gen­er­al Psy­chi­a­try.

Two im­por­tant brain struc­tures ap­pear smaller than av­er­age in men who are long-time, heavy pot smok­ers, a stu­dy has found. (Im­age cour­tesy Okla. Dept. of Nar­cot­ics)


There’s con­flict­ing ev­i­dence re­gard­ing ma­ri­jua­na’s long-term ef­fects, they not­ed, and light use “may not lead to sig­nif­i­cant neu­ro­tox­ic ef­fects.” But “these re­sults sug­gest that heavy daily use might in­deed be tox­ic to hu­man brain tis­sue.”

The users in the study had smoked at least five “joints” daily for 10 years or more. Mu­rat Yü­cel of the Uni­ver­s­ity of Mel­bourne, Aus­tral­ia, and col­leagues took brain scans of 15 such men, and com­pared the re­sults to those of 16 non-users.

They found that with­in the brain, the hip­pocam­pus—thought to reg­u­late emo­tion and mem­o­ry—and the amyg­da­la, in­volved with fear and ag­gres­sion, tended to be smaller in the pot smok­ers. The av­er­age size re­duc­tion was 12 and 7 per­cent, re­spec­tive­ly.

Al­so, users per­formed “sig­nif­i­cantly worse” than others on ver­bal learn­ing tests, though this did­n’t cor­re­late to the size of brain struc­tures, the re­search­ers wrote.

“With nearly 15 mil­lion Amer­i­cans us­ing can­na­bis in a giv­en month, 3.4 mil­lion us­ing can­na­bis daily for 12 months or more and 2.1 mil­lion com­menc­ing use eve­ry year, there is a clear need to con­duct ro­bust in­ves­ti­ga­t­ions” of its ef­fects, they added.


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Two important brain structures appear smaller than average in men who are long-time, heavy pot smokers, a study has found. The findings challenge a popular view that marijuana is harmless, wrote the authors of the study, published in the June issue of the research journal Archives of General Psychiatry. There’s conflicting evidence regarding marijuana’s long-term effects, they noted, and light use “may not lead to significant neurotoxic effects.” But “these results suggest that heavy daily use might indeed be toxic to human brain tissue.” The users in the study had smoked at least five “joints” daily for 10 years or more. Murat Yücel of the University of Melbourne, Australia, and colleagues took brain scans of 15 such men, and compared the results to those of 16 non-users. They found that within the brain, the hippocampus—thought to regulate emotion and memory—and the amygdala, involved with fear and aggression, tended to be smaller in the pot smokers. The average size reduction was 12 and 7 percent, respectively. Also, “users performed significantly worse than controls on verbal learning,” though this didn’t correlate to the size of brain structures, the researchers wrote. “With nearly 15 million Americans using cannabis in a given month, 3.4 million using cannabis daily for 12 months or more and 2.1 million commencing use every year, there is a clear need to conduct robust investigations” of its effects, they added.