Hmmm...
It's a really small sample size.
I find myself wondering if the change in brain structure is a result of all the pot use or if a pre-existing condition spurred on a heavy addiction.
And that's not even taking into consideration other possible factors. Do these men have steady jobs, access to health care, and proper nutrition? Are there any other recreational substances they've enjoyed? Most *heavy* pot smokers I know have trouble holding down a job, tend to also drink heavily, and have experimented with other drugs.
Oh, the article answered one of my questions. "The heavy marijuana users, average age 40, said they had used other illicit drugs less than 10 times, the researchers said."