TUESDAY, March 14, 2006 (HealthDay News) -- Adding a new twist to the debate over the effects of marijuana, Greek researchers say the minds of long-term pot smokers don't process things as well as those of other people.
People who smoked at least four joints a week performed worse on a variety of mental tests, including those measuring memory, attention and verbal fluency. And those who smoked for more than 10 years had the most problems of all, the researchers report in the March 14 issue of Neurology.
The study doesn't definitively link marijuana use to the cognitive problems, and it's possible that other factors could be to blame. Prior research has also offered conflicting findings about the mental effects of marijuana.
Still, the findings suggest that chronic pot smoking isn't a good idea over the long term, said Barbara Flannery, a drug and alcohol researcher who's familiar with the findings. "The longer you smoke marijuana, the more likely you are to experience a diminution of cognitive functions that are critical for 'normal' daily functioning," she said.
Greek researchers at the University Hospital of Patras gave cognitive tests to 64 people in a drug-abuse treatment program. Twenty were long-term marijuana users who had smoked for at least a decade. Twenty others had smoked pot for shorter periods and the remaining 24 hadn't used marijuana for at least two years. All the participants abstained from using marijuana for 24 hours prior to the tests.
Marijuana users tended to do worse on the tests, the researchers report. On a decision-making test, for example, long-term users were impaired 70 percent of the time, compared to 55 percent among short-term users and 8 percent among non-users.
In another test, the participants were asked to recall a list of words. The non-pot users did best, recalling an average of 12 of 15 words; the long-term users could only remember an average of seven.
Research into the mental effects of marijuana is controversial, with some advocates of the drug claiming that its rumored mind-addling powers are a myth.
The new study brings new information to the debate, especially about the differences between short- and long-term pot users, said researcher Flannery, who works for a private research company affiliated with the University of North Carolina in Research Triangle Park, N.C., said it's hard to "untangle" whether marijuana use directly causes brain deficits.
The study's lead author agreed that conclusions on cause-and-effect are difficult. However, researcher Lambros Messinis added that his team tried to adjust the study to account for other possible causes.
If marijuana does harm the brain, it's not clear how it may do so. Scientists have found evidence that regular marijuana use can constrict blood vessels in the brain -- an effect also found in people with diabetes and chronic high blood pressure.
The good news? There's some evidence that it may be possible to reverse the mental effects of long-term marijuana use, Messinis said, although not all experts agree that this might be possible.
More information
To learn more about drug addiction, try the National Institute on Drug Abuse.
