A major concern with long-term use of cannabis, whether consumed for recreational or medicinal purposes, is the possibility of irreversible damage to brain structure or function. There is agreement among researchers, however, that long-term use of cannabis does not result in structural brain damage or gross cognitive deficits (Solowij, 2002). However, the possibility that cannabis use results in subtle and/or specific cognitive impairments has been the subject of considerable controversy.
Hall and Solowij (1998) have reported detrimental neurocognitive effects in long-term heavy cannabis users. However, these subjects were investigated while they were current users and therefore the deficits could have been a direct effect of intoxication. Until recently, no studies were available which observed a period of abstinence of more than a few days (Solowij, 2002). In an additional study performed by Solowij and colleagues (2002), very long-term cannabis users (median 24 yr of use) showed deficits in tests for memory and attention. However, users defined as short-term users, but whose mean duration of use was nevertheless 10.2 yr, did not differ, in general, from controls. Moreover, abstinence intervals were minimal (median 17 h). As recovery of function is likely to involve changes in plasticity, such as CBj receptor densities, such a short interval is unlikely to allow for significant reversals.
More recently, investigations have been performed which allowed for considerably longer abstinence periods (1 to 3 months). These investigations in general do not support the existence of a residual effect of cannabis that persists well after cessation of consumption. Thus, in a long-term prospective investigation of a sample of children of mothers who smoked cannabis during pregnancy (Fried et al., 2002), these children themselves became heavy users as young adults, displaying a 4-point decrease in IQ (as compared to an earlier assessment made at the age of 9-12 yr). In contrast, heavy users who had abstained from cannabis use for at least 3 months gained IQ points, which was similar to the findings in light current users or nonusers: these groups gained 3.5, 5.8, and 2.6 points respectively. In a study by Pope and colleagues (2001), a 28-d-long wash-out period was sufficient to eliminate the deficits in neuropsychological performance observed in current heavy users. It is possible, however, that a more detailed analysis of cognitive function would reveal residual effects of cannabis smoking. Perhaps future studies will more definitively answer this question.
In accordance with the latter two investigations, a recent meta-analytic study of residual neurocognitive effects of cannabis use did not reveal detrimental effects, except for a modest impairment in memory tests. The authors conclude that when medical use of cannabinoids is being considered, the health benefits should be carefully weighed against the very modest potential for cognitive decline (Grant et al., 2003).
A meta-analytic study of 3206 subjects on the behavioral traits associated with cannabis use and abuse (Gorman and Derzon, 2002) demonstrated merely an association with "unconventionality" and not more serious disorders, mirroring much older observations (Goode, 1970). Cannabis use and good mental health are not mutually exclusive. A recent British study has shown that drug experimentation is associated with high self-esteem (Regis, 2001). Another British study showed no causal relationship between cannabis use and delinquency in young people (Hammersley et al., 2003).
In short, chronic cannabis use does not appear, after cessation of drug use, to result in a significant cognitive decline or behavioral symptomatology. However, as discussed for cannabis use as a risk factor for schizophrenia (see section titled 'The Last 40 Years"), age of onset of cannabis use may be critical in determining the outcome of cannabis consumption later in life. Thus, in a recent study by Pope and colleagues (2003), heavy cannabis users who had started before the age of 17 differed from controls in neuropsychological tests, mainly on verbal IQ.
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