Regular pot smokers have shrunken brains, study says

From here:

Experimental mice have been telling us this for years, but pot-smoking humans didn't want to believe it could happen to them: Compared with a person who never smoked marijuana, someone who uses marijuana regularly has, on average, less gray matter in his orbital frontal cortex, a region that is a key node in the brain's reward, motivation, decision-making and addictive behaviors network.

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Researchers noted that the IQ of the marijuana-using group was significantly lower than that of the non-using group — not a finding of the study, but an incidental factor that might be indirectly linked to marijuana use.

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