This review considers the effects of cannabis on cognitive functioning, in both short and long term. Although the general impression supported by many studies is that cannabis causes cognitive decline, particularly with long-term usage, some research suggests that this may not be the case. Nevertheless, certain specific neuropsychological parameters have been found to be affected. Most commonly and consistently reported are response time, prolongation of word viewing time, basic oculomotor deficit, residual verbal memory and executive functioning. The pathways to cognitive dysfunction are given particular focus, including the role of the central nervous system (CNS) cannabinoid system. Finally, the psychiatric effects of cannabis are considered in light of the idea that cognitive function may be the common denominator in the association between cannabis and psychotic disorders.
Cannabis is one of the most commonly abused illicit drugs. The World Health Organization reports that almost 3% of the world's adult population abuses cannabis, with many more individuals reporting less frequent use. Adolescents in particular consume high levels of cannabis, starting generally between 12 and 16 years of age. This is an important factor from a psychiatric and developmental point of view. Cannabis use is of important consideration in light of its recognized acute and long-term health effects. Active compounds of cannabis, called cannabidols, have 64 active isomers, each having different effects on human health and behavior. Only one metabolite, tetrahydrocannabinol (THC), is reported to be an active metabolite responsive for its effects. Studies show that it induces both psychical and physical dependencies, but the perception of withdrawal is weak on account of its slow elimination. There is a widely held belief that cannabis is inert to the brain, and although the psychological consequences are quite evident, the population at large seems unconvinced. There is much debate about the nature of cannabis dependence, as it is considered non-addicting due to the absence of a withdrawal state. This presumption has also been proved wrong.
The general impression supported by many studies is that cannabis causes cognitive decline, particularly with long-term usage. Majority of studies have suggested a significant cognitive decline in cannabis abusers compared to non-abusers and healthy controls. A report by Bartholomew et al. suggested that cannabis use has a detrimental effect on prospective memory ability in young adults but users may not be aware of these deficits. Cannabis is known to produce substantial acute effects on human cognition and visuomotor skills. Many recent studies additionally revealed rather long-lasting effects on basic oculomotor control, especially after chronic use. Even so, it is still unknown to what extent these deficits play a role in everyday tasks that strongly rely on an efficient saccade system, such as reading.
Cannabis has a negative impact on cognition; however, the current body of research literature does not provide evidence of significant, long-term effects due to cannabis use. Several acute effects are noted and some are suggestive of negative mental health consequences. Evidence from both animal and human studies suggests that the severity of the effects of cannabis use on cognitive development is dependent on the age when cannabis use begins. One possible explanation is that those who begin cannabis use early in adolescence are more likely to become heavily dependent. It is plausible that chronic cannabis abuse will then interfere with educational and vocational training. From a more biological perspective, however, use of cannabis during critical developmental periods in the still maturing brain may induce persistent alterations in brain structure and brain function. Therefore, the effects of frequent cannabis use during adolescence could be different from…