Researchers led by Lambros Messinis at the University Hospital of Patras in Greece tested the mental abilities of 20 long-term users who had taken marijuana heavily. They smoked at least four joints a week for an average of 15 years, reported the online edition of New Scientist.
Their brains were rustier than those of 20 short-term users who had averaged seven years of use and 24 who had used the drug sporadically or not at all.
Long-term users performed worse in tests to measure memory, learning ability and the capacity to recall information.
Asked to recall lists of 15 words that they had seen earlier, the long-term users averaged seven, compared with nine recalled by short-term users and 12 by controls.