Consumption As a psychoactive substance, cannabis has various cognitive effects at the point of use, which generally fade within a few hours. With heavy, long-term use, more permanent cognitive effects may result, although the consensus on this within the scientific community is shaky.
Acute cognitive effects of cannabis
The acute cognitive effects of cannabis are numerous and vary widely from person to person according to several factors including genetic makeup, age, gender, and individual tolerance.
It is believed that the phytocannabinoid ??-THC is primarily responsible for the cognitive effects of cannabis, as it the only compound contained within cannabis that is known to have a psychoactive effect; however, it is thought that various other cannabinoids including CBD work in synergy with THC to mediate its cognitive effects, dependent on ratio and dose.
The best-known acute cognitive effects associated with cannabis are generally related to memory, learning, and higher executive function. In practical terms, this can mean that while under the influence of cannabis, an individual’s ability to plan, make decisions, maintain attention, solve problems, retain information, and cope with unexpected situations may be compromised to some extent.
Research into the acute cognitive effects of cannabis
An important review of the existing literature published in 2012 notes that researchers first began investigating the acute cognitive effects (those experienced from 0-6 hours after consumption) of cannabis use in the 1970s, and have found consistent evidence of alterations to learning and memory function. However, results of research into cannabis’ effects on higher executive function have been less conclusive.
The effect of cannabis on attention is controversial, and appear to depend greatly on individual tolerance. A 2001 study (Hart et al.) found that neither high or low doses of THC impaired response to attentional tasks compared to placebo in chronic cannabis users, and that high doses of THC actually significantly improved performance on a visual tracking task. Conversely, a 2009 study (Morrison et al.) found that infrequent cannabis users experienced significant impairment in attention and concentration in the THC-using group compared to controls.
The acute effect of cannabis on decision-making and risk-taking is also somewhat controversial, and appears to depend on dose. A 2005 study (Lane et al.) compared the effect of either high or low doses of THC compared to placebo, and found that subjects exposed to high doses exhibited significantly greater risk-taking compared to the low-dose or placebo groups. Conversely, a 2006 study (Raemakers et al.) concluded that the effect of THC on risk-taking was negligible, although it significantly lengthened the time taken to make decisions, and increased the rate of incorrect responses in the tests taken by subjects.
Memory is impaired by acute cannabis intoxication, and has been demonstrated to do so on multiple occasions, in research papers spanning decades. An early study (Miller et al, 1977) concluded that overall ability to recall information was reduced in subjects exposed to cannabis, and that visual recall was inferior to verbal; more recently, the aforementioned 2001 (Hart et al.) study demonstrated that memory was impaired in a dose-dependent manner in subjects exposed to THC.
Residual & permanent cognitive effects of chronic cannabis use
Long-term users of cannabis may be at a somewhat elevated risk of certain cognitive impairments that persist beyond the cessation of intoxication. These impairments may be residual (occurring not long after cessation of intoxication, and related to lingering traces of the active compound in the system) or permanent (persisting after all traces of the substance have left the system); however, existing research has thus far yielded inconsistent and occasionally contradictory results.
A 2002 US study (Solowij et al.) found that long-term users of cannabis demonstrated significant impairment in memory and learning tasks compared with short-term users or non-users. Conversely, a more recent study (Tait et al, 2011) found that although baseline results indicated significant differences between cannabis-using and non-using groups, when these results were controlled for gender and educational level, these differences all but disappeared.
However, after adjustment it was found that former heavy users of cannabis performed better on one of four cognitive tests (specifically, one related to verbal learning) than current heavy users, indicating that some degree of cognitive impairment exists in heavy, long-term users (but also that such impairments improved in former users).
Clearly, the extent of cannabis use plays a role in the potential development of long-term cognitive impairment. A 2006 study (Jager et al.) concluded that although cognitive effects may occur in heavy long-term users, moderate users displayed no deficits in memory or learning, although they did exhibit altered activity in the left superior parietal cortex of the brain during working memory processing.
Effect of abstinence on cognition in chronic users
As previously mentioned, the 2001 (Hart et al.) study showed that chronic cannabis users performed less well on attention tasks when abstinent compared to performance after ingestion of THC. This phenomenon has been observed repeatedly: a 2004 study (Kelleher et al.) compared heavy, chronic cannabis users with non-cannabis using controls, and found that abstinent users exhibited significantly slower information-processing speed, which normalized after smoking cannabis.
The study authors observe that this may imply that former cannabis users are at risk of resuming cannabis use in an attempt to regain optimal information-processing abilities. There have been several other studies indicating that abstinent regular users continue to experience cognitive impairments for some time subsequent to cessation of use, but it does appear that abstinent chronic users may expect to regain optimal cognitive function with time.
Cognitive effects in former cannabis users
A 2002 review of the existing research concludes that while cognitive impairment is apparent at days 0, 1 and 7 in heavy users, by day 28 most studies found that any residual impairment was negligible.
As a 2003 review of the literature also observes: “Our results indicate that there might be decrements in the ability to learn and remember new information in chronic users, whereas other cognitive abilities are unaffected. However, from a neurocognitive standpoint, the small magnitude of these effect sizes suggests that if cannabis compounds are found to have therapeutic value, they may have an acceptable margin of safety under the more limited conditions of exposure that would likely obtain in a medical setting.”
However, a more recent study (Bolla et al, 2005) found that chronic cannabis users that had been abstinent for 25 days exhibited significant neurocognitive impairment compared to non-cannabis using controls, as well as demonstrating altered brain activity—abstinent users showed increased activity in the left cerebellum and decreased activity in the right lateral orbitofrontal cortex and right dorsolateral prefrontal cortex.
This effect was also shown to be dose-dependent, as it was more pronounced in the ‘heavy use’ group compared to the ‘moderate use’ group when results were further subdivided. Furthermore, the moderate group differed only slightly from the control group, leading the authors to speculate that cognitive impairment in chronic users is dependent on a threshold-dose effect. Thus, it appears that further research is needed to ascertain the precise nature of the effect.
Cannabis, cognition and mental illness
The potential effects of cannabis use for individuals susceptible to or already suffering from mental illnesses such as schizophrenia is a vast area of research that is impossible to comprehensively summarize without overlooking a huge amount of detail. However, the mainstream media consistently disregards this fact and supplies incomplete (and often entirely inaccurate) information to the general public.
For the last few years, much has been made of the apparent link between cannabis use and onset of psychosis (an abnormal mental state that can lead to schizophrenia, bipolar disorder, or various other psychiatric conditions—or can resolve without causing long-term problems). It is certainly the case that individuals suffering from schizophrenia are far more likely to use cannabis than the general public; however, it appears that this cannabis use is likely to ameliorate symptoms rather than contribute to them.
A 2010 meta-analysis of the existing research found that schizophrenics that used cannabis actually exhibited superior cognitive and neuropsychological functioning compared to non-cannabis using schizophrenics, and demonstrated only selective impairments while non-users demonstrated impairments across the board.
Cannabis users performed better on working memory, visual memory and executive function; furthermore, individuals who began using cannabis earlier in life performed better on tests than those that began use later in life. These findings strongly suggest that, rather than triggering the disease, cannabis use in prodromal schizophrenics (i.e. individuals in the very early stages of schizophrenia) in fact confers a neuroprotective effect.
Cannabis & cognition in adolescents
Although it appears that cannabis use early in life is not responsible for the onset of psychosis or schizophrenia, there remains widespread concern that adolescents who use cannabis are subjecting themselves to various cognitive and psychological effects. Indeed, a 2014 review of the literature observed that “evidence is building to suggest that regular cannabis use during the teenage or emerging adult years (typically ages 15–25 years) is associated with cognitive deficits”.
The vast majority of the studies reviewed reported cannabis-related cognitive deficits including impaired processing speed, verbal memory, executive function and complex attention, along with an increased propensity for risky behaviour. Studies investigating the effect of abstinence on adolescent users found that impaired processing speed, verbal memory, sustained attention and sequencing ability persisted at one month’s abstinence, but markedly improved at four months. However, this particular review made no mention of the methodology used to find and categorize the studies it analyzed.
It is important to note that many of the existing studies into the cognitive effects of cannabis are methodologically weak, and the existence of apparently-conflicting results may in many cases be due to a lack of differentiation on the basis of age, gender or other confounding factors.
In future, as the weight of research grows, it will be increasingly possible to identify flaws in study design and thus ascertain with far greater accuracy the likely effects of cannabis on the individual cognitive response, and how individual makeup can affect the severity and duration of cognitive impairment.
Furthermore, most of the existing research focuses solely on THC, but as our understanding of the synergistic effects of combining phytocannabinoids in specific ratios increases, we are likely to identify myriad cognitive effects—such as the fact that combining CBD with THC entirely negates the memory-impairing effect.