Background: An important question that arises in policy discussions concerning the use of cannabis is whether the possible harmful effects of cannabis are reversible.
Aim: To review studies on residual neuropsychological, structural and functional brain abnormalities that may have resulted from the long-term use of cannabis.
Method: When we searched Medline (January 1966 - December 2003) and EMBASE (January 1988 - December 2003) using the key words 'cannabis', 'marijuana', 'neuropsychological test', 'cognition', 'CT-scan', 'MRI', 'PET', 'SPECT' and 'brain', we found 29 studies in our area of interest. We selected only those neuropsychological and functional brain imaging studies in which the patients had a controlled abstinence period longer than 98 hours.
Results: On the basis of six studies we concluded that there was insufficient evidence to prove conclusively that long-term cannabis use causes or does not cause residual abnormalities. The results of our review were also inconclusive whether cannabis use during adolescence may have a lasting effect on cognitive functioning and brain structure. However, we could not rule out the possibility that (1) certain cognitive and cerebral abnormalities existed in patients before cannabis use began and (2) that patients were suffering from subacute effects of cannabis.
Conclusion: So far, research into the residual effects of cannabis use has been inadequate. Future studies should concentrate on the effect of cannabis use during early adolescence. More insight should come from longitudinal studies involving neuropsychological measurements and brain imaging before cannabis use begins. The abstinence period should be at least seven weeks.