There is little doubt that using Cannabis can impair cognition acutely, “after all, this is the basic reason for its recreational use,” as one author wrote.29 As with psychosis, the available evidence indicates that the degree of cognitive impairment is related to the frequency and duration of Cannabis use as well as age of onset of use.30,31
Few studies have assessed cognitive functioning in relation to Cannabis potency with most only examining the effects of relatively low-potency Cannabis with inconsistent results. For example, 2 studies compared cognitive performance in individuals smoking Cannabis with 1.8% and 3.9% THC. One study found that using higher potency Cannabis resulted in prolonged time needed to complete certain cognitive tasks,32 whereas the other found greater impairment in performance on a decision-making task at both potencies compared with non-users but no differences between the 2 dosages.33 Detecting significant differences may be difficult within the narrow range of low Cannabis potency studied where any findings have limited applicability in the context of today’s Cannabis with much higher THC content.
To date, only 1 study has assessed cognition at higher Cannabis potencies, comparing Cannabis with 4% THC to 13% THC.34 Cognitive impairments increased with higher potency, especially in tasks that measured motor control and executive functioning. Therefore it appears that higher potency Cannabis use is associated with greater acute cognitive impairment.
The longer-term effects on cognition are less clear, with conflicting evidence about whether Cannabis use can result in residual cognitive impairment despite abstinence.30,35 A recent review concluded that “the magnitude of neuropsychological impairment and the extent to which it persists after abstinence may depend on the frequency and the duration of Cannabis use, length of abstinence, and age at onset of use.”31 The effects of HPC on long-term cognitive deficits have not been studied.
Structural brain changes
A number of studies have determined an association between Cannabis use and brain changes involving structures governing memory and emotional processing, including reduced volume of the hippocampus,36 temporal cortex, insula, and orbitofrontal cortex.37 Although many of these changes appear to be dose-related, some morphologic changes have been reported among young recreational users without Cannabis dependence.38 This has resulted in an understandable concern about the effects of Cannabis on the brains of young people with limited exposure; however, it is not yet clear to what extent detected brain changes are pathological and reflect functional deficits.
Recent research using newer neuroimaging modalities provides preliminary support of Cannabis use associated with white matter changes that, in turn, are correlated with cognitive impairment.39 One study comparing low-potency Cannabis and HPC users with and without first-episode psychosis found a significant effect of Cannabis potency on disturbances in white matter microstructural organization in the corpus callosum.40 These findings provide sufficient cause for concern that structural brain changes associated with cognitive impairment are more likely to occur with HPC use.