Human BrainScience & Tech

The Blunt Truth: Weeding Out the Myths about Cannabis and Cognition

If the controversial debate over cannabis use has left you feeling hazy, you aren’t alone. Public perceptions of drugs and drug policies (specifically concerning the decriminalization of cannabis) have evolved dramatically since the original 1987 broadcast of the iconic anti-drug PSA that compared your brain on drugs to a frying pan of sizzling sunny-side-up eggs.  In preparation of the upcoming legalization of cannabis for recreational use in 2017, a society once guided by the conservative values propagandized by cautionary tales like the 1936 film Reefer Madness has seemingly increased its tolerance of cannabis consumption.

So, what’s the burning issue? Scientific research has shown the adverse effects associated with medicinal, religious, spiritual, and recreational use of cannabis. These include (but are not limited to) addiction, cognitive impairment, and potentially, a heightened risk of psychosis – proving that the grass truly isn’t always greener…

However, to its 182 million users around the globe, three letters offer a promise of pleasure, euphoria, escape, and relief from pain: P-O-T. When it comes to six other letters – THC (or Δ9-tetrahydrocannabinol) and CBD (or cannabidiol) – public awareness and familiarity go up in smoke. These compounds are two of approximately one hundred different cannabinoids that are responsible for cannabis’s unique functional properties.

The drug’s major psychoactive ingredient, THC, diminishes one’s capacity to learn new information and promotes anxious and psychotic behaviours. Conversely, CBD has the opposite effect (such as learning enhancement, and anti-psychotic and anti-anxiety effects). When people smoke cannabis, they inhale the two ingredients at once. Thus, the CBD may mitigate the unfavourable consequences of the THC. 

However with today’s cannabis, this cancelling out may not occur. Compared to the strains of cannabis that were obtainable decades ago, today’s cannabis show significant differences in composition.  THC levels in illegally purchased cannabis have increased tremendously, while CBD content is now nearly insignificant. These changes have caused researchers to question how applicable the findings of older longitudinal cohort studies are to the health and wellbeing of modern-day cannabis users.

Short-term Effects

The acute responses of cannabis use occur upon intoxication (for example, the temporary sensation of feeling “stoned”). Regardless of its method of administration, a mere dose of THC causes working and episodic memory impairments – specifically, the encoding and subsequent retrieval of new memories – in a dose-dependent manner. The onset of these effects occurs more immediately through inhalation or intravenous injection, rather than oral ingestion. As demonstrated by extensive preclinical research, certain areas of the brain involved in memory (such as the hippocampus, amygdala, and prefrontal cortex) contain high densities of cannabinoid receptors.  THC interferes with plasticity of hippocampal neurons involved in learning and memory; long-term potentiation and long-term depression become deregulated, resulting in changes in the strength of synaptic transmission.

Scientific evidence suggests that the specific strain of cannabis used may affect the acute memory-related responses of cannabis intoxication. It appears that high-CBD strains of cannabis defend against THC-induced acute memory impairments, with CBD minimizing conditioned fear responses, as well as protecting against THC-related hippocampal volume reduction. The long term effects of chronic exposure to CBD are suggested to be the focus of future research.

Long-term Effects

Researchers have experienced difficulties determining the causal relationships between cannabis consumption and long-term mental health. Numerous confounding variables, such as baseline cognitive function prior to cannabis use, drug and alcohol interactions, and age when drug use commenced impact study findings.  Even so, scientists have found that the most commonly reported long-term responses to cannabis consumption – impairments in new episodic memory encoding – may persist for days after drug administration, and typically occur in frequent, heavy cannabis users. 

Currently, research related to working memory, attention and impulsivity under the influence of cannabis is inconclusive. In addition, the ability to make accurate decisions is inversely correlated to the number of symptoms of cannabis addiction a user exhibits. In both adult and adolescent cannabis users, chronic consumption was associated to alterations in the structure and function of certain regions of the brain (primarily the medial temporal and frontal cortices, as well as the cerebellum). These changes – often manifesting in a user’s compromised inhibitory control and decision making – may propagate cannabis addiction further, even in spite of negative repercussions associated with drug use. Researchers hypothesize that the risk of addiction development may significantly increase as a result of the interactions between these neurocognitive alterations and specific genetic components.

Age-related Effects

Studies suggest cannabis influences cognitive function by affecting the developmental processes of neural circuits prevalent during adolescence, resulting in age-related effects. When individuals begin using cannabis during their adolescent years, neurological structure and function become severely impaired compared to their adult counterparts, causing deficits in visuospatial attention, verbal fluency, and inhibition. Adolescents who consume cannabis frequently have reduced white-matter integrity, affecting their neurocognitive performance. Correlations exist between cannabis addictions that begin during adolescence and persist throughout adulthood, an IQ decline of approximately 8 points, and lower educational attainment (dropping out of school, poorer academic performance).

As cannabis becomes more accessible and the stigma associated with consumption shatters, the population of cannabis addicts may increase.  Gaining a comprehensive understanding of the factors that predispose individuals to addiction and developing preventative strategies to counter these aspects have become most urgent subjects of research.


  • Jordyn Posluns

    Jordyn is a recent graduate from the University of Guelph's Biomedical Sciences program, and currently works for Bell Canada Supply Chain.

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