People who regularly smoke marijuana may find their memories growing hazy over time, a study published Monday suggests.
In a study of long-term and shorter-term marijuana users, researchers in Greece found that both groups performed more poorly on tests of memory, attention and other cognitive abilities than a comparison group who'd only occasionally used the drug.
Long-term users -- who'd smoked four or more joints per week for at least 10 years -- showed the greatest deficits.
The findings, published in the journal Neurology, add to the conflicting body of research on the effects of marijuana on the brain. While many studies have suggested that long-time pot smoking dulls memory, attention span and mental acuity, some have found no large differences in these skills between marijuana users and non-users.
One recent analysis of 15 studies found only minor effects on memory among long-time pot users, and no clear effect on attention, language, reasoning and a number of other cognitive functions.
One problem is that it's difficult for studies such as the current one to establish a definite cause-and-effect relationship between marijuana and intellectual deficits, Dr. Lambros Messinis, the study's lead author, told Reuters Health.
Though the researchers accounted for a number of variables -- like education, use of other drugs and the presence of clinical depression -- it's tough to control for all the factors that could make heavy marijuana users different from other people, according to Messinis.
Still, he and his colleagues say, their findings are in line with certain past studies linking heavy, long-term pot smoking to "subtle" deficits in intellectual abilities.
The study included 40 marijuana users ages 17 to 49 who were in a drug abuse treatment program; all had used the drug frequently for at least five years, but half -- those considered long-term users -- had smoked for 10 years or more. They were compared with 24 adults the same age who had used pot no more than 20 times in their lives.
Overall, both long- and shorter-term marijuana users performed more poorly on tests of memory, attention and mental-processing speed. The proportion of study participants deemed "impaired," according to the researchers, was highest in the long-term group and lowest in the comparison group.
Long-time pot users showed the greatest problems on tests where they were asked to learn and remember a series of words. They were "significantly" below the published norms for these tests, according to Messinis and his colleagues.
It's not yet clear whether the intellectual deficits linked to marijuana are lasting, Messinis said, but research "generally supports" the notion that these problems are reversible after longer periods of abstinence. People in his study were required to have been abstinent only for the 24 hours before taking the tests.
Another unknown, according to Messinis, is whether marijuana use at a young age may affect the brain differently than use during adulthood. Knowledge in this area, he said, is still "poor."