The short-term and long-term effects of cannabis on cognition: recent advances in the field

Introduction

Recent global changes in cannabis legislation parallel increases in use and decreases in harm perception. Yet, there is still little conclusive evidence on the effects of cannabisuse. This review specifically focuses on the effects of cannabis use on cognition. Cognition encompasses our thoughts and shapes our behaviour, and refers to distinct but partially overlapping processes such as learning, memory, attention, inhibition, decision-making, and emotion regulation. Cannabis contains over a hundred different cannabinoidsincluding Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD). Although the mechanisms are unclear, cannabinoids like THC and CBD potentially affect cognition through interactions with the endogenous cannabinoid system in the brain. This system in-turn regulates many other neurotransmitter systems including the dopamine system often implicated in substance use disorders (SUD). Moreover, like in other SUDs, the development of Cannabis Use Disorder (CUD) may also be related to pre-existing cognitive deficits. Given the rapidly developing evidence base, we will discuss the most recent evidence for the effects of cannabis intoxication (short-term) and heavy cannabis use (almost daily use, long-term) on cognition (Table 1). We thereby start with basal cognitive functions, moving towards more complex cognitive functions and the role of affective processes therein.

Table 1. Summary of current evidence for short-term and long-term effects of cannabis on cognition

Cannabis and cognition: current knowledge and recent advances

Learning and memory

Cannabis intoxication impairs learning and memory in a dose-dependent manner, although significant individual differences exist. Studies in heavy cannabis users are less consistent, but learning and immediate recall deficits are most commonly reported in active cannabis users. A recent longitudinal study in adolescent cannabis users suggests a causal link between cannabis exposure and immediate, but not delayed recall in an episodic memory task. Furthermore, another recent study showed that trial-by-trial verbal learning rates were slower in cannabis users compared to controls, and these learning rates were associated with altered functionality of the parahippocampal gyrus, thalamus and midbrain regions. While altered feedback processing may play a role in learning deficits observed in alcohol and other substance users, this may not necessarily be the case in cannabis users. Furthermore, impairments may not be relegated to only memory of real experiences. Kloft et al. showed that cannabis intoxication increased susceptibility to false memory, an effect that appeared most prominent at immediate compared to delayed recall.

Subacute intoxication effects likely contribute to the described effects in cannabis users. The effects of cannabis on memory performance and related alterations in brain activity fade with abstinence. In line with this, working memory performance and functionality of the underlying brain network was only found to be impaired in individuals with a positive urine screen for THC. Despite the heterogeneous and potential timebound nature of the observed deficits, cannabis use-related learning and memory problems could seriously impact daily functioning of heavy cannabis users, including performance in school or at work. A combination of psychological, neurological, and neurobiological research is crucial to further elucidate the apparent complexity of mechanisms underlying the effects of cannabis on memory.

Attention

Similar to learning and memory, cannabis intoxication consistently results in a THC-dose-dependent reduction of the capacity to orient attention towards task-relevant stimuli. In heavy compared to occasional cannabis users, tolerance to the acute effect of cannabis on attentional control was related to reduced responsiveness of the reward system after intoxication. This may relate to the general tolerance to cognitive impairments by cannabis intoxication often observed in heavy users. Heavy cannabis users also develop an attentional bias towards cannabis and related objects that may interfere with other attentional processes. Although effect sizes were small, a recent meta-analysis showed evidence for an attentional bias towards cannabis-related words and pictures in heavy cannabis users. Attentional bias has been linked to the severity of CUD and might reflect an involuntary early perceptual bias, supported by increased amplitude and earlier peak of the N1 component in response to distracting cannabis stimuli.

Inhibition

Cannabis use, and drug use in general, has often been associated with poor inhibitory control. With regards to motor inhibition, cannabis intoxication consistently and dose dependently reduces the ability to inhibit an ongoing motor response, as measured with the stop-signal task. In contrast, inhibition before a response is initiated, as measured with the go/no-go task, may not be impaired by intoxication. Findings on the effects of heavy cannabis use on motor inhibition are less consistent. However, aside from potential problems caused by impairments in motor control due to cannabis intoxication, motor inhibition might not well-reflect the daily life inhibition problems present in most substance users. Indeed, slower proactive inhibitory control-related processes, such as those measured with the classical Stroop were found to relate to cannabis craving.

Decision-making

More complex cognitive functions such as decision-making heavily rely on the integrity of the basal cognitive functions discussed above and deficits in any of those might in turn result in risky decisions like substance use. The complexity of the processes involved may explain the inconsistent findings on the effects of cannabis intoxication and heavy use on decision-making. Nonetheless, progress has been made and recent studies provide new insight into how heavy cannabis use and the context in which decisions are made affect risky decision-making. For example, a recent study on financial delay discounting (preferring immediate small rewards over delayed bigger rewards) observed a positive relationship between increased delay discounting and frequency of cannabis use. Interestingly, Gilman et al. found that heavy cannabis using adolescents compared to controls differed on risk taking in the social, safety, and ethical domains, but not the financial domain. In general, risky decision-making in heavy cannabis users seems associated with increased sensitivity to immediate gain accompanied by decreased loss sensitivity.

The importance of context and emotion

The previously discussed findings highlight the need for a more fine-grained investigation of cognitive subprocesses and their interactions, as well as the importance of the context in which cognition is measured. While cannabis use by a popular peer may bias decision-making in an occasional user, for individuals with a CUD, decision-making may be particularly compromised when confronted with cannabis-related cues. As with attentional bias, cannabis-related cues may also activate an approach bias towards cannabis in heavy cannabis users. Moreover, acute stress may influence cognitive performance. For example, acute stress affects prospective memory performance in both heavy cannabis users and controls, but the effects are larger in heavy cannabis users. On the other hand, increased working memory capacity seems to protect heavy cannabis users from craving under stressful circumstances. Taken together, potential cognitive deficits in heavy cannabis users may manifest themselves depending on contextual factors.

The impact of cannabis use on emotion processing is an important factor to consider herein. Although data is limited, cannabis intoxication may negatively affect emotion recognition. This seems to be most apparent for negative emotions and appears to be related to reduced brain activity in reward and cognitive control related brain areas when presented with negative faces. A recent study focusing on gender differencesidentified complex interactions between gender and cannabis use patterns in relation to the early processing of emotional stimuli (EEG, ERP: P1 and P3). This highlights the general importance of assessing gender differences in the effects of cannabis use. This is a particularly relevant issue in the domain of emotion processing research because of the high rates of comorbidity between cannabis use and disorders associated with emotion processing (e.g. anxiety) and the commonly reported gender difference in the prevalence of these disorders.

Field wide difficulties and future directions

Aside from the classic confounders such as polysubstance use and comorbid mental healthproblems, as well as a lack of longitudinal data limiting our understanding of the causal relationship between cannabis and cognition, cannabis research is facing significant difficulties which have been brought to attention by the majority of recent reviews on the topic. While overcoming these difficulties is of utmost importance, clear solutions are still lacking.

First, the vast majority of studies on the long-term effects of heavy cannabis use on cognition share one confounding factor: the abstinence period. Studies show that THC metabolites are detectable in the plasma of heavy cannabis users for over a week and even longer detectability is possible due to THC’s lipophilic characteristics. In line with this, cannabis-use-dependent neurocognitive impairments can be detected for as long as 28 days after cessation. Hence, studies in current heavy cannabis users struggle to differentiate subacute from long-term effects. Although this confound should be acknowledged and more wide-spread assessment of THC metabolites is warranted, subacute effects should not always be seen as a problem in itself. After all, the mix of acute, subacute, and long-term effects represent what a current heavy cannabis user is dealing with in daily life. Nevertheless, more knowledge of the potential for recovery after abstinence and the role of CUD severity in recovery is needed.

Second, problems with quantifying use are often reported and pose a true problem for comparability across studies. Variable definitions of heavy cannabis use and the lack of standard cannabis units are recurrent problems. While both problems might reflect semantics, and defining categories for frequency and heaviness of use might indeed primarily require discussion, developing a standard unit is extremely complicated. Recently, attempts were made to develop a standard unit of cannabis, but the complexity and variability in cannabis products and routes of administration hampers practicality. Cannabis contains over a hundred different types of cannabinoids and the THC:CBD ratio differs significantly between region and even between batches. Poor knowledge about exposure history in most studies complicates research even further. To improve our knowledge base, accessible and more reliable methods to quantify cannabis use are needed. However, even then, research in most countries heavily relies on changes in local legislation to allow for these methods to be used.

Third, there are methodological problems that plague comparability in systematic reviewsand meta-analyses. While increasing the amount of research will increase the power of these types of reviews, studies are rarely replicated and the variability between measures to assess the same cognitive construct remains a problem. An increase in power will not reflect an increase in knowledge when this heterogeneity problem is not solved. In line with this, it remains important to be aware of the risks of assuming that similar tasks measure the same construct like is often done when aggregating results from stop-signal and go/no-go task.

Finally, it may be that the effects of heavy cannabis use on cognition are indeed mixed. The same dose of THC may result in impairments in some, while leading to improvement in others. These individual differences are likely to depend on a large variety of moderating factors including THC:CBD ratio, differences in THC metabolization, poly-substance use, severity of cannabis dependence, age of onset, gender, and mental health. In turn, the combined effects of these factors might vary with the context under which cannabis is consumed and cognition is assessed.

Conclusion

The rapid increase of research into cannabis and its effects on cognition has provided us with answers as well as questions. While there is increasing evidence that cannabis intoxication negatively affects basal cognitive functions like episodic memory, attentional control, and motor inhibition, results on the long-term effects of heavy cannabis use, and potential recovery after abstinence, remain equivocal for most cognitive domains. Despite a slow start, cannabis research is breaking ground. Nevertheless, field-wide difficulties in quantification, methods of measuring cognitive constructs, and the influence of subacute effects seriously hamper the road ahead and require attention now. Multidisciplinary collaboration and investment in studies that solve these problems should be prioritized.

Summary

Recent alterations in cannabis legislation worldwide correlate with increased usage and diminished perceptions of associated harm. However, conclusive evidence regarding cannabis' effects remains limited. This review focuses on cannabis' impact on cognition, encompassing processes like learning, memory, attention, inhibition, decision-making, and emotion regulation. Cannabis comprises numerous cannabinoids, including THC and CBD, which potentially modulate cognition through interactions with the brain's endogenous cannabinoid system. This system influences various neurotransmitter systems, including the dopamine system, implicated in substance use disorders. The development of Cannabis Use Disorder (CUD) may also be linked to pre-existing cognitive deficits. This review examines the most recent evidence on the effects of cannabis intoxication (short-term) and heavy cannabis use (long-term) on cognition.

Learning and Memory

Cannabis intoxication impairs learning and memory in a dose-dependent manner, though individual differences are substantial. Studies on heavy users show inconsistencies, but learning and immediate recall deficits are commonly reported in active users. A longitudinal study suggests a causal link between adolescent cannabis exposure and immediate, but not delayed, recall. Another study demonstrates slower verbal learning rates in cannabis users, correlating with altered parahippocampal gyrus, thalamus, and midbrain activity. While altered feedback processing may influence learning deficits in other substance users, this may not apply to cannabis users. Furthermore, impairments extend beyond memories of real experiences; intoxication increases susceptibility to false memories, particularly at immediate recall. Intoxication effects often subside with abstinence, with working memory and associated brain network function impaired only in individuals with positive THC urine screens. Cannabis-related learning and memory problems can significantly impact daily functioning in heavy users. Further research integrating psychological, neurological, and neurobiological perspectives is needed to elucidate the underlying mechanisms.

Attention

Cannabis intoxication consistently reduces the ability to focus attention on relevant stimuli in a THC-dose-dependent manner. In heavy users, tolerance to acute attentional effects is linked to reduced reward system responsiveness post-intoxication. This may relate to the general tolerance to cognitive impairments often observed in heavy users. Heavy users also develop an attentional bias towards cannabis-related objects, potentially interfering with other attentional processes. A meta-analysis indicates an attentional bias towards cannabis-related stimuli in heavy users. This bias correlates with CUD severity and may represent an involuntary perceptual bias, as suggested by altered N1 component responses to distracting cannabis stimuli.

Inhibition

Cannabis use is often associated with poor inhibitory control. Intoxication consistently reduces the ability to inhibit ongoing motor responses, as measured by the stop-signal task. However, inhibition before response initiation (go/no-go task) may not be impaired. Findings on heavy use effects are inconsistent. While motor inhibition might not fully reflect daily-life challenges, slower proactive inhibitory control, as measured by the Stroop task, correlates with cannabis craving.

Decision-Making

The complexity of decision-making processes may explain inconsistent findings on cannabis effects. Studies show a positive correlation between increased delay discounting and cannabis use frequency. Heavy-using adolescents exhibit altered risk-taking in social, safety, and ethical domains, but not financial domains. Risky decision-making in heavy users seems linked to increased sensitivity to immediate gains and decreased loss sensitivity.

Context and Emotion

Research highlights the need for detailed investigation of cognitive subprocesses and contextual factors. Cannabis-related cues might activate approach bias in heavy users. Acute stress affects prospective memory in both heavy users and controls, but more significantly in heavy users. Increased working memory capacity may protect heavy users from craving under stress. Cognitive deficits may manifest differently based on context. Cannabis intoxication may negatively affect emotion recognition, particularly negative emotions, potentially due to reduced activity in reward and cognitive control brain areas. Studies reveal complex interactions between gender and cannabis use patterns in emotional stimuli processing. This underscores the importance of considering gender differences in cannabis use effects, particularly given the high comorbidity between cannabis use and emotion-processing disorders.

Field-Wide Difficulties and Future Directions

Cannabis research faces significant challenges, including polysubstance use, comorbid mental health issues, and limited longitudinal data. The abstinence period confounds studies on long-term effects, as THC metabolites remain detectable for extended periods, making it difficult to distinguish subacute from long-term effects. Standardizing cannabis usage quantification is problematic due to variable definitions of heavy use and lack of standard units. Methodological inconsistencies between studies hamper comparability in systematic reviews and meta-analyses. The effects of heavy cannabis use on cognition may be highly variable depending on multiple factors.

Conclusion

Research on cannabis and cognition reveals both answers and new questions. While intoxication negatively affects basal cognitive functions, long-term effects and recovery after abstinence remain unclear. Challenges in quantification, measurement methods, and subacute effects hinder progress. Multidisciplinary collaboration and targeted research are crucial to advance the field.

Summary

Recent global shifts in cannabis legislation coincide with increased use and diminished perceptions of harm. However, conclusive evidence regarding the effects of cannabis use remains limited. This review focuses on the impact of cannabis use on cognition, encompassing processes like learning, memory, attention, inhibition, decision-making, and emotion regulation. Cannabis contains numerous cannabinoids, including THC and CBD, which may influence cognition via interactions with the brain's endogenous cannabinoid system and other neurotransmitter systems. The development of Cannabis Use Disorder (CUD) might also be linked to pre-existing cognitive deficits. This review examines the short-term effects of cannabis intoxication and the long-term effects of heavy cannabis use on cognition.

Learning and Memory

Cannabis intoxication impairs learning and memory in a dose-dependent manner, although individual differences exist. Studies on heavy cannabis users show less consistency, but learning and immediate recall deficits are frequently reported in active users. A longitudinal study suggests a causal link between cannabis exposure and immediate recall deficits in episodic memory tasks. Impairments extend beyond memories of real experiences; cannabis intoxication increases susceptibility to false memories. These effects often diminish with abstinence, highlighting the role of subacute intoxication. Cognitive deficits related to cannabis use can significantly impact daily functioning. Further research integrating psychological, neurological, and neurobiological perspectives is needed to clarify the complex mechanisms at play.

Attention

Cannabis intoxication consistently reduces the ability to focus attention on relevant stimuli. Heavy users may develop tolerance to the acute effects on attentional control. They also exhibit an attentional bias toward cannabis and related items, potentially interfering with other attentional processes. A meta-analysis confirmed an attentional bias toward cannabis-related stimuli in heavy users. This bias has been linked to CUD severity and may represent an involuntary perceptual bias.

Inhibition

Cannabis intoxication consistently reduces the ability to inhibit ongoing motor responses, but the impact on response initiation may be less pronounced. Findings on heavy cannabis use are inconsistent. However, slower proactive inhibitory control processes correlate with cannabis craving, suggesting real-world implications beyond simple motor tasks.

Decision-Making

The effects of cannabis on decision-making are inconsistent, likely due to the complexity of the processes involved. Studies show a relationship between increased delay discounting (preferring immediate smaller rewards) and cannabis use frequency. Risky decision-making in heavy users appears associated with increased sensitivity to immediate gains and decreased sensitivity to potential losses.

The Importance of Context and Emotion

Cognitive deficits in heavy cannabis users may vary depending on context. Cannabis-related cues may activate an approach bias. Acute stress can affect prospective memory more significantly in heavy users. However, higher working memory capacity may buffer against stress-induced craving. Cannabis intoxication might negatively affect emotion recognition, particularly for negative emotions, potentially related to reduced brain activity in reward and cognitive control areas. Gender differences in emotional stimuli processing further complicate the picture.

Field-Wide Difficulties and Future Directions

Cannabis research faces challenges including polysubstance use, comorbid mental health problems, and limited longitudinal data. The abstinence period in studies on heavy users confounds the differentiation between subacute and long-term effects. Inconsistent definitions of "heavy cannabis use" and a lack of standardized cannabis units hinder comparability across studies. Methodological inconsistencies in assessing cognitive constructs also limit the value of systematic reviews and meta-analyses. Individual responses to cannabis vary significantly, depending on factors such as THC:CBD ratio, metabolism, poly-substance use, and mental health.

Conclusion

While cannabis intoxication clearly impairs some cognitive functions, the long-term effects of heavy use remain unclear. Difficulties in quantifying cannabis use and methodological inconsistencies hamper research progress. Multidisciplinary collaboration and investment in research addressing these issues are crucial for a clearer understanding of cannabis's effects on cognition.

Summary

Recent changes in cannabis laws have led to increased use and a lessened perception of its risks. However, the effects of cannabis on cognition—the mental processes involved in thinking and behavior—aren't fully understood. This includes learning, memory, attention, decision-making, and emotional control. Cannabis contains many cannabinoids, including THC and CBD, which may affect cognition by interacting with the brain's endocannabinoid system. This system influences neurotransmitters like dopamine, which is linked to substance use disorders (SUDs). The development of Cannabis Use Disorder (CUD) might also be connected to pre-existing cognitive weaknesses. This review examines the short-term (intoxication) and long-term (heavy, near-daily use) effects of cannabis on cognition.

Learning and Memory

Cannabis intoxication worsens learning and memory, depending on the dose, though individual differences are significant. Studies on long-term, heavy users show less consistency, but deficits in learning and immediate recall are common among active users. A longitudinal study of adolescents suggests that cannabis use causes problems with immediate recall, but not delayed recall, in episodic memory tasks. Another study found that cannabis users learned verbal information more slowly than non-users and that this was linked to altered activity in brain areas like the parahippocampal gyrus, thalamus, and midbrain. While altered feedback processing might explain learning problems in some substance users, this may not apply to cannabis users. Furthermore, impairment may not be limited to memories of real events; cannabis intoxication may increase susceptibility to false memories. The effects of cannabis on memory seem to lessen with abstinence.

Attention

Cannabis intoxication reduces the ability to focus on important information, depending on the THC dose. Heavy users may develop tolerance to this effect, which may relate to reduced responsiveness of the reward system. Heavy users also develop an attentional bias towards cannabis and related items, potentially interfering with other attentional tasks. A meta-analysis showed evidence of this bias in heavy users. This bias has been linked to CUD severity and may be an involuntary, early perceptual response.

Inhibition

Cannabis intoxication consistently impairs the ability to stop an action already in progress, as shown in stop-signal tasks. However, the ability to prevent an action from starting, as measured by go/no-go tasks, might not be affected. The long-term effects of heavy use are less consistent. Proactive inhibition problems, like those measured by the Stroop test, are linked to cannabis cravings.

Decision-Making

Cannabis's effect on decision-making is inconsistent, likely due to the complexity of the process. However, studies show that more frequent cannabis use is related to increased preference for smaller, immediate rewards over larger, delayed rewards. In adolescents, heavy cannabis use is linked to increased risk-taking in social, safety, and ethical situations, but not financial ones. Risk-taking in heavy users seems to involve increased sensitivity to potential gains and decreased awareness of potential losses.

Context and Emotion

Research highlights the need to consider the context in which cognitive abilities are assessed. Cannabis-related cues may trigger approach biases in heavy users. Stress can affect cognitive performance, particularly prospective memory. Working memory capacity may protect against cannabis cravings under stress. Cannabis intoxication might impair emotion recognition, particularly negative emotions, linked to decreased activity in brain regions associated with reward and cognitive control. Studies are also starting to consider gender differences in how cannabis use impacts emotion processing.

Research Challenges

Cannabis research faces challenges including polysubstance use, comorbid mental health issues, and a lack of longitudinal data. Many studies on long-term effects are confounded by the abstinence period, because THC metabolites remain detectable for a considerable time. Variable definitions of heavy use and the lack of standardized units for cannabis hinder research comparability. Methodological inconsistencies between studies create problems for reviews and meta-analyses. Finally, individual responses to cannabis vary, potentially depending on THC:CBD ratio, metabolism, poly-substance use, cannabis dependence, age, gender, and mental health.

Conclusion

Research shows that cannabis intoxication negatively impacts basic cognitive functions. The long-term effects of heavy use and the possibility of recovery after abstinence need further study. Improving cannabis use quantification, standardizing cognitive testing methods, and accounting for sub-acute effects are crucial. Multidisciplinary collaborations and funding for research addressing these issues are vital.

Summary

Many places are changing their laws about cannabis, and more people are using it. Scientists are trying to figure out how cannabis affects our brains. Cannabis has many different chemicals, including THC and CBD. These chemicals can affect how we think, learn, and remember things. This article looks at how cannabis affects our brains in both the short-term (right after using it) and long-term (if you use it for a long time).

Learning and Memory

Using cannabis can make it harder to learn and remember things. The more cannabis used, the worse it gets for some people. Studies show that people who use a lot of cannabis often have trouble remembering things right away. Some children and adolescents who use cannabis have trouble remembering things, but this may improve later. It's important to remember that everyone is different.

Attention

Cannabis can also make it harder to pay attention. If cannabis is used a lot, an individual might get used to this effect. People who use a lot of cannabis may also focus more on things related to cannabis. This can make it harder to concentrate on other things.

Inhibition

Cannabis can make it harder to stop individuals from doing things. This is especially true for things they are already doing.

Decision-Making

Cannabis can affect our decision-making. People who use a lot of cannabis may make riskier choices, especially if they are focused on getting something immediately.

The Importance of Context and Emotion

How cannabis affects your brain depends on many things. For example, if you're stressed, it might affect your memory even more. Also, if you see or think about cannabis, it might make you want it more. Cannabis might also affect how we understand other people's feelings.

Field-Wide Difficulties and Future Directions

Scientists are still learning a lot about how cannabis affects the brain. One problem is that it's hard to tell the difference between the effects of cannabis right after using it and long-term effects, because the effects can last for a long time. It's also hard to measure exactly how much cannabis people use, as there are many different kinds of cannabis. It's also hard to compare studies because scientists use different tests to measure the same thing. Finally, some people are more affected by cannabis than others.

Conclusion

Scientists are learning more about how cannabis affects our brains. Using cannabis can make it harder to learn, remember, pay attention, and make good decisions. More research is needed to understand the long-term effects and how to measure cannabis use more accurately.