Adolescent Marijuana Use: Effects On Cognitive Functioning

Introduction

Marijuana was first legalized for recreational use in the state of Colorado in 2014 [1,2]. Since then, 24 states have legalized the recreational use of marijuana [1]. This has reduced the perceived risks of the drug among the younger populations: a survey done in 2021 found that 78.4% of U.S. high school seniors believed regular marijuana use was not harmful [3]. Believing the drug is safe can lead to increased use. In 2022, 30.7% of U.S. high school seniors reported using marijuana in the last year [4]. The significant number of adolescents using marijuana is problematic because this stage is a critical time for cognitive development. Marijuana poses a threat because its psychoactive component, tetrahydrocannabinol (THC), is capable of passing the blood-brain barrier and thus acting directly on the central nervous system [5, 6]. From the age of 12 into young adulthood, the amygdala, prefrontal cortex, and hippocampus are changing, and neural connections are being strengthened [7, 8]. These areas are associated with memory, learning, attention, and inhibitory control and are threatened by marijuana use [6,9]. Current literature’s proposed long-term cognitive effects of marijuana use on these key functions in adolescence are based on recurring correlations found in studies. However, a lack of consistent and reliable approaches to control for confounding variables remains an issue. This review will focus on the current state of research on the effects of adolescent marijuana use on cognitive function, its potential long-term consequences, and highlight the inconsistencies in research methodologies. 

Effects on Learning and Memory

Working memory is the short-term storage of information used in learning, reasoning, and comprehension [10]. This short-term storage is fundamental in decision-making and complex reasoning, abilities required to successfully complete learning-based tasks in an academic setting. This in turn plays critical roles in processing information and making educated decisions. As a result, the cognitive functions associated with working memory are often targets of research. Academic settings often exercise one manifestation of working memory, verbal learning skills, in students. Verbal learning includes material that can be read (i.e. letters, numbers, and symbols) and is tested by memorizing and recalling new material [10, 11]. 

The relationship with adolescent-marijuna use and declined verbal learning has been an area of focus in research. A study done by Blest-Hopley et al. (2021) compared 42 individuals, half of which had smoked marijuana during adolescence, while the other half was abstinent. It was found that brain activity during verbal learning exercises is impeded in those who smoked marijuana frequently compared to sober adolescents. Frequent users showed disrupted parahippocampal and midbrain function, which are associated with slower-learning abilities and impaired memory [12, 13]. Reduced verbal activity associated with early marijuana use was also shown in a longitudinal study performed by Ren and Fishbein (2023). This five-year study followed 10-12 year olds with no history of any substance use prior to the start of the study. Some of these participants took up marijuana use within the study period and self-reported the frequency of use. Assessments on verbal learning were performed annually. Results showed that at the end of the five year study, participants who engaged in marijuana use had slightly reduced verbal learning abilities compared to abstinent participants [6]. These results demonstrate the declining cognitive effects due to marijuana use over time by following participants over years, whereas the single-day study led by Blest-Hopley did not have the participants baseline to compare their results to. Baseline refers to the participants’ performance on assessments prior to the start of the study. Blest-Hopley and her team used sober adolescents as control participants rather than measuring the baseline of participants that may engage in marijuana use, which may not be as accurate as performing a longitudinal study due to variation in the way individuals learn and perform tasks. Despite these differences in study methods and designs, Blest-Hopley et al. (2021) and Ren and Fishbein (2023) both agree that adolescent marijuana use impairs verbal learning and memory. 

Effects on Attentional and Inhibitory Control

Attentional control is the ability to concentrate on a stimulus from the environment. It is involved in learning, conversations, and comprehension [10]. Impaired attentional control often leads to difficulty in making decisions, hyperactivity, and decreased inhibitory control. Inhibitory control is the ability to understand what behaviors are acceptable in various situations and the consequences of those behaviors, ultimately being able to resist inappropriate outbursts [14, 15]. Attentional and inhibitory control often go hand-in-hand and are a focus in current research due to their importance in an academic setting. 

Understanding how attentional and inhibitory control work in the context of impairment may shed light on the effects of marijuana on cognition. A longitudinal study by Paige and Colder (2020) followed 11-14 year olds for seven years with the participants’ attentional and inhibitory control assessed annually. The data collected was compared to expected adolescent developmental trajectories based on the sober participants. The researchers found that high marijuana use in participants between the ages of 12 to 14 was significantly correlated with decreased attentional and inhibitory control in these participants six years later. However, this correlation was weaker in inhibitory control [15]. Declined attentional control was also found by Murray et al. (2022), which compared neural electroactivity in adolescents, ages 18-20,  and adults, ages 30-40, under the influence of specified doses of THC using an electroencephalogram (EEG). Reaction-test exercises were performed to stimulate the brain regions involved in attentional control. Their results found that neural electroactivity was disrupted in adolescents but not in adults when given the same dose of THC [16]. This is significant because it shows a concrete effect relating to declined cognitive functioning in adolescents. The study illustrates the sensitivity of the developing brain to marijuana and how the neural activity in areas associated with attention can be disrupted. The question of exactly how continued neural electrical disruption affects cognitive development remains, although there are studies currently investigating this relationship. One such study by Demirayak et al. (2023) found an association between reduced neural electrical activity related to attention and mild cognitive impairment in stimuli processing [17]. This may point to a potential mechanism behind the reduced attentional control found in adolescent marijuana users. More research in this area in the context of marijuana-impaired cognition is required to understand the long-term effects of disrupted brain activity. 

Long Term Academic Consequences 

Together, these studies elucidate the potential effects of both acute and chronic adolescent marijuana use on learning. Impaired attentional and inhibitory control have both been associated with decreased mathematical and spelling abilities [14], while reduced inhibitory control was also associated with increased academic dishonesty in undergraduate students due to the desire for instant gratification and the reduced ability to consider the consequences [18]. Overall, chronic marijuana use is associated with a lower GPA and lower academic motivation [14, 19]. As a result, young individuals using marijuana are at increased risk of falling behind academically and may be less likely to pursue higher education [20]. Low academic achievement may limit individuals’ career opportunities and thus their economic status. The reduced cognitive function associated with adolescent marijuana use and its subsequent impacts on academic success therefore pose a significant threat to the futures of young users, and a stronger understanding of the correlation can help encourage public support in educating youth on the risks associated with marijuana use.  

Shortcomings and Differences in Methods

It is difficult to determine the exact potency of the marijuana that participants self-reported. THC content, of which potency is based on, can change depending on what part of the plant is used, how it was cultivated, and how the product was stored after harvest (UN). THC content in marijuana has dramatically increased in the past decade, from 8.9% to 17.1%, likely due to new methods in extracting THC from marijuana plants [22]. There has yet to be a method used in experimental literature that accounts for the potency of the marijuana participants used. 

Moreover, baseline conditions of participants’ substance use and cognitive state are not equally measured between studies. For example, Noorbakhsh et al. (2020) and Paige and Colder (2020) only accounted for alcohol and marijuana consumption in the participants surveyed and failed to measure their baseline cognitive functioning. By ignoring the impacts of other substances, it may become unclear how much of the results attributed to marijuana is actually due to marijuana. This depends on how such substances interact with marijuana and thereby affect the accuracy of studies, however, and existing evidence provides inconsistent answers. For example, a study by Ren and Fishbein (2023) did account for other substances and found the same results as Paige and Colder (2020), while another by Schaefer et al. (2021) found the effects of marijuana use on cognitive function to reduce once other drugs are accounted for. Similarly, ignoring baseline cognition can skew the results, showing a false strong correlation between declined function and marijuana use. Current studies lack the evidence to prove direct causality between adolescent marijuana use and declines in cognitive functioning [19]. Therefore, it is crucial that baseline cognitive functioning is measured in participants in order to gain a stronger understanding of marijuana’s effect on cognition. Moving forward, baseline cognitive functioning should be included in standard methods in order to provide stronger evidence of the effect marijuana has on adolescent cognitive functioning. 

Conclusion

This review sought to show the current state of research pertaining to adolescent marijuana use on cognitive functioning. It has shown consistent findings between studies on the effects of learning, memory, attention, and inhibition. Several opportunities exist for further research, such as focusing on the impact of disrupted neural electroactivity on cognitive development and marijuana’s effect on inhibitory control. Current experimental studies do show correlation, however the evidence is insufficient to establish causation due to issues in accounting for confounding factors such as other substance use and pre-existing cognitive impairment. Developing a standard of accounting for potential confounding factors between studies can help establish more concrete results. Further developments towards establishing causation would allow for a greater understanding of the risk marijuana use poses to adolescent cognitive development and potential consequences on educational outcomes.

About the Author: Lauren Prince

Lauren is a class of 2024 Biological Sciences major.

Author's Note

I wrote this literature review in UWP 102B for my scientific peers. I decided to focus my review on the long term effects of smoking cannabis during adolescence because cannabis is a commonly used substance and is becoming increasingly popular among minors. I was interested in researching if cannabis affects the cognitive development of adolescents, and if so, how the effects may manifest in the future of young users. I hope readers leave with a greater understanding of the risks associated with smoking cannabis at a young age and approach the substance with greater caution.

References

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