Cannabis acutely impairs cognitive performance so it is possible that chronic use may cause longer-lasting cognitive impairment. This possibility seemed to be supported by clinical observations in the USA during the early 1970s (e.g. Kolansky and Moore, 1971) that young adults who had used cannabis weekly or more often had poor attention span, poor concentration, and confusion (Kolansky and Moore, 1971). More recently, long-term cannabis users seeking help to stop using cannabis report impaired memory and thinking (Stephens et al, 2000). The difficulty with these reports has been in ruling out the possibilities that these cognitive impairments preceded cannabis use or that they were due to other drug use.

Cross-cultural studies

One research strategy in searching for any cognitive effect of cannabis has been to assess the cognitive performance of heavy cannabis users in cultures with a tradition of such use. An early report by Soueif (1971) illustrates the problems with this strategy. Soueif studied Egyptian male prisoners of whom 850 were hashish smokers and 839 controls. The hashish users performed more poorly than the controls on 10 out of 16 measures of cognitive performance (Soueif, 1971) but the two groups differed in other ways that may have affected cognitive performance, namely, the hashish users were less well-educated and more likely to use opiates and alcohol than the controls (Carlin, 1986).

In the late 1960s three cross-cultural studies were conducted in Jamaica (Rubin and Comitas, 1975), Greece (Kokkevi and Dornbush, 1977) and Costa Rica (Satz et al., 1976) to assess the effects of chronic cannabis use on cognitive functioning. It was assumed that any cognitive effects of chronic daily cannabis use would be apparent in users with a long history of heavy cannabis use, a pattern that was common in these cultures.

These cross-cultural studies users provide equivocal evidence of subtle cognitive impairment among long-term cannabis users. Some studies failed to find any differences (e.g. Rubin and Comitas, 1975) and the others found modest cognitive impairment in persons with a long history of heavy cannabis use (Page et al., 1988).