Cannabis is the most common substance of abuse across the world, with more than 130 million people estimated to be using cannabis (Crean, Crane & Mason, 2011). Many conflicting suggestions have been put forward regarding the effects of cannabis on health. The most common postulates relate to its ability to “kill brain cells, harm brain function, and cause learning and cognitive impairment” (Solowij & Battisti, 2008, p. 81). A number of studies have demonstrated that chronic users of this illicit substance show disordered encoding, storage, alteration, and retrieval of information (Solowij & Battisti, 2008). Although it is widely accepted that cannabis causes intoxication and compromises short term memory, the long term effects as a result of cannabis use continue to generate immense controversy because scientific studies on these effects are contradicting. This escalates the debate concerning the long term implications of cannabis use on memory, even after cessation of use (Medina et al., 2007).
The effects of cannabis on the central nervous system are brought about by its chemical substance called tetrahydrocannabinol (THC). THC is responsible for the euphoric and relaxation feelings associated with cannabis use because it acts on the brain reward system. Upon inhalation of TCH, it rapidly diffuses from the pulmonary tissue into the circulation, finding its way into the brain tissue (Ranganathan & D’Souza, 2006). THC binds to and stimulates specific receptors in the body called cannabinoid receptors.
The naturally occurring receptors are fundamental in enhancing the developmental process of the CNS and its various roles (Ranganathan & D’Souza, 2006). When the THC binds to cannabinoid receptors, they become activated and cause interference of normal functioning of brain regions, such as the cerebellum and hippocampus. The cerebellum is involved in maintaining balance, posture, and coordination of movement, while the hippocampus is responsible for memory formation. Cannabis is, therefore, directly involved in the disruption of motor activity of the brain. This, consequently, affects the ‘short-term memory’ negatively with time.
The impact of frequent cannabis use on long term memory has not been well studied. Controversy, therefore, exists over the relationship between cannabis use and chronic memory impairment. In addition, it is also not clear whether people who cease using cannabis following heavy use can be able to regain some cognitive abilities (Lyons et al., 2004). Some studies have shown that chronic users gradually recover their cognitive abilities even after quitting cannabis. However, other studies have also shown that the effects of cannabis on the brain often accumulate and reduce critical functional skills over time, more so in people with mental disorders or due to the normal aging process (Hart et al., 2010).
Studies to demonstrate the effect of cannabis on the brain have indicated that the linkages between nerve cells in the brain hippocampus become severed when mice are administered with THC (Morgan & Zimmer, 1995). The hippocampus is crucial in memory development, both new and experienced memory. It is believed that THC modifies the way information is processed in the hippocampus (Yucel et al., 2008). Cognitive impairments in animals administered with THC have been correlated with structural and functional alterations in the hippocampus, especially if the exposure occurred during early developmental stages. This possibly implies that cannabis use affects both short-term and long-term memory (Ilan, Smith & Gevins, 2004).
Cannabis use over time impairs memory and learning to the extent that the effects are felt way after ceasing to use the drug (Crean, Crane & Mason, 2011). The effects have been shown to be more pronounced in people who started using the substance as adolescents, and the effects may continue for many years (Crean, Crane & Mason, 2011). It has been demonstrated that regions that play the role of memory and learning have their connections severely impaired as a result of the chronic use of cannabis (Yucel et al., 2008).
This supports the suggestion that frequent use of cannabis, especially by young people, can lead to chronic effects on the structure and function of the brain. A study by Jacobsen et al. (2004) showed a reduction in IQ by close to 8 points among persons who had consumed cannabis in their teenage years by the time they reached 48 years. The participants were studied prospectively. People who started using cannabis as adults, however, did not show a considerable decline in their intelligence quotient levels. Further, it has been reported that the lost cognitive abilities are not recovered upon cessation of use (Jacobsen et al., 2004).
In spite of the recognized effects of cannabis use on memory, some studies on the long-term use of cannabis show that it does not lead to major harm on long-term memory. Jager et al. (2006) carried out ‘functional magnetic resonance imaging’ studies on cannabis users by examining structural alterations in the hippocampal region. The researchers found out that cannabis users expressed reduced activation compared to non-users in brain areas concerned with associative learning, especially in the hippocampal areas and the dorsolateral prefrontal cortex.
Further analysis by Jager et al. (2006) showed no variations in brain tissue structure between the two groups. The researchers did not establish any correlation between hippocampal tissue structure and the degree of brain functioning in the hippocampal region, leading to the conclusion that decreased brain activation does not necessarily imply neurocognitive damage; instead, it may reflect a non-cognitive effect associated with regular marijuana use, such as alterations in cerebral perfusion.
Morgan and Zimmer (1995) discredit the suggestions that the use of cannabis causes chronic memory loss, as reported in experiments on rhesus monkeys, which revealed structural alterations of the brain. Morgan and Zimmer (1995) argued that the doses required to cause such changes in humans cannot be achieved even among heavy cannabis users, implying that cannabis does not cause physiological damage to brain cells. The authors point that using cannabis in large amounts can interfere with an individual’s potency to relay novel information for long-term storage. Structural damage can also occur permanently. This assumption has also been supported by Block et al. (1999), whereby they observed no regional changes in the grey or white matter through magnetic resonance imaging studies on the brain tissue of frequent cannabis users. The researchers, nevertheless, indicated that the MRI technique used could not reveal possible structural damage at the microscopic level, making the findings unclear.
Lyon et al. (2004) also demonstrated that even after genetic variability is controlled for, cannabis use does not cause significant long-term residual effects on memory. These findings were similarly reflected in a study by Pope et al. (2001). The study investigated the neuropsychological functioning of chronic users of cannabis. This study revealed that cannabis users experienced some cognitive difficulties following heavy use, but the deficits were temporary and were attributed to the recent use of cannabis instead of cumulative effects. These findings have also been postulated by Schreiner and Dunn (2012) in a meta-analysis study that showed that cannabis use does not cause permanent memory impairment.
While cannabis use can lead to memory impairment, it is important that various factors be considered in order to determine whether it can permanently impair memory. Given that comorbidities are frequently associated with substance use, it is worth establishing whether those people who develop chronic memory impairments had cognitive impairments prior to cannabis use (Lyketsos et al., 1999). In addition, most chronic users of cannabis are likely to abuse other drugs that may also have long-term neurological impacts (Lyketsos et al., 1999). Besides, most of the studies regarding chronic memory impairment due to cannabis use are experimental and may be subject to limitations such as small study subjects, insufficient background data on the confounding variables, or presence of cognitive and personality disorders (Messinis et al., 2006).
In conclusion, a huge body of knowledge exists regarding the potentially harmful impacts of cannabis on brain cells, memory, and brain function. However, studies reveal contradicting information regarding the effects, in particular, whether the use of cannabis can cause permanent memory impairment. The findings discussed show that most chronic users of cannabis exhibit some form of memory or brain impairment. Nevertheless, there is still insufficient evidence to conclusively link cannabis use as a direct cause of such memory impairments. In addition, studies on the structure of the brain have not revealed any difference between chronic users and non-users, although it has been indicated that deviations may occur at molecular levels. It is, therefore, difficult to definitively conclude that there is chronic memory impairment as a result of cannabis use.
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