As political winds shift and the world enters into an age appreciative of medical cannabis, it\’s critical to understand the effects this plant has on the human brain. Undeniably, cannabis impacts human cognition. Cannabinoids (and more specifically THC) cause changes to perception and affect cognitive performance and behavior. Yet, today, we know surprisingly little about the ramifications of these effects. 

The short-term effects of cannabis on cognition are familiar to most and discussed within the scientific literature. It\’s the long-term effects on cognition that remain challenging to assess. Studies are contradictory, and what\’s more, much of the research comes from an anti-cannabis perspective reminiscent of the War on Drugs. 

As cannabis becomes a legitimate aspect of the medical landscape, are the risks to cognitive performance worth the medicinal benefits? What do we know about its impact on the brain, cognition, and behaviours, both over the short term and the long?

Short-Term Effects of Cannabis on Cognition

Pop culture continues to portray chronic cannabis users as slow, unmotivated, and largely apathetic. Many of these lingering stereotypes stem directly from the short-term effects that the plant produces. 

Even within the scientific community, there are long-held ideas about cannabis\’ ability to reduce drive and goal-oriented behaviour. In 1968, these assumptions about cannabis consumers were given credence by W.H. McGlothlin and L.J. West, who went so far as to propose cannabis amotivational syndrome. While this proposed syndrome didn\’t catch on, similar assumptions about the short-term effects of cannabis continue to this day. 

Cannabis, in particular products and strains containing tetrahydrocannabinol (THC), has acute short-term effects. As summarized in Marijuana and Madness (2012), \”Cannabis induces perceptual distortions and impairs memory and concentration during acute intoxication.\” While this summary is generally well accepted among medical research, cannabis can (and frequently does) impact some people much differently. As the authors note, functional impairment depends on experience with cannabis, dose size, and more.

During the acute intoxication phase, the primary impairments include attention, learning, short-term memory, executive function, abstract thinking, and decision-making. The severity of the impairment is linked to the dose size, familiarity with THC, and tolerance. Researchers theorize that impairment is mitigated by tolerance and cannabis experience because our minds create \”compensatory strategies during acute intoxication to facilitate performance that might otherwise be impaired.\” 

These immediate intoxicating effects following the consumption of THC-laden products are short-lived. As soon as a few hours after consumption, most people find these effects have subsided — which initially sounds like great news for cannabis\’ future in medicine. If THC has only short-term impacts on cognition and memory, patients have little to worry about, even with long-term use. But what happens when cannabis is used daily, for months (if not years) for chronic conditions? What are the long-term impacts on cognition?

Lingering Long-Term Effect of Cannabis on Cognition

For patients and recreational users classified as heavy and long-term consumers, does cannabis cause neurological alterations? Do any of the impacts on cognitive performance continue, even after a period of abstinence?

The authors behind Marijuana and Madness studied cannabis\’ effects on the human brain. Although inconsistent at times, the research does suggest that chronic cannabis consumption impairs inhibitory processing, which is the process of maintaining one thought while at the same time inhibiting another (the Stroop Task is a classic example of this). Long-term use may also have prolonged effects on executive function. 

In 2016, the researchers of \”Keep off the grass? Cannabis, cognition and addiction\” asked how cannabis consumption impacts I.Q. For the first question, the authors assessed the results of large prospective cohort studies, which all focused on young adults and adolescents. Only one of the studies reported a negative impact on I.Q. score. While there were several challenges to these studies (retrospective self-report of cannabis use, the risks of adolescent use, and the value of an I.Q. test to capture intelligence), they suggest that I.Q.outcomes are unrelated to long-term cannabis use.

If we go back to the stereotype of the sleepy, unmotivated and lazy cannabis consumer, does the research support this? In \”Effects of Cannabis Use on Human Behavior, Including Cognition, Motivation, and Psychosis: A Review,\” also published in 2016, \”there is preliminary laboratory evidence supporting an association between reduced motivation for reward-related behavior in cannabis users compared with control individuals.\” 

Chronic use may impact motivation because it affects the dopamine system. Heavy long-term use disrupts dopamine signalling, and in many respects, dopamine signalling sustains motivation. 

The literature hasn\’t reached a firm conclusion about cannabis and cognition, but there are nevertheless serious concerns with what has been published so far. Chronic consumption reduces motivation, inhibitory processing, and executive function. 

Lessons from Long-Term Impacts on Cognition for Medical Cannabis

While it is still far too early to know precisely how cannabis impacts memory, executive function, and motivation, physicians and patients may want to note the early findings. Long-term use of cannabis, even when consumed as a medicine, could possibly lead to ongoing cognitive impairments. During sustained use, patients may discover their motivation, memory, and/or executive function foggier than normal.

The good news? Cognitive effects caused by heavy cannabis use dissipate after four to six weeks of abstinence. One study, summarized in Keep off the Grass, used positron emission tomography imaging on a group of chronic cannabis users, before and after cessation. 

Within four weeks, the brain imaging signalled (through downregulation of cortical CB1Rs correlated with years of use) a return to control levels. More recent studies have found some instances of CB1R density returning to control levels within as little as two days.

Cannabis Affects Cognition, But Only During Ongoing Use

As with all other medicines, both patients and physicians must weigh the short- and long-term effects of cannabis before incorporating it into a treatment plan. Yes, these effects may disappear following a period of abstinence, but are they acceptable for the patient during treatment?

Many patients wrongly assume cannabis is completely free of side effects. But, while the cognitive impairments may not be as outwardly noticeable or destructive as those from many, more conventional pharmaceuticals, they are still worthy of study and risk assessment. 

We need much more unbiased research on cannabis and cognition to determine precisely what\’s going on. How do dose size, treatment duration, and cannabinoid profile impact memory, learning, and other cognitive processes?