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This free science article teaches students about the peer-reviewed research about the potential effects of cannabis and—more precisely—delta-9 THC on memory and cognitive function.
Does Cannabis Affect Memory?
Cannabis consumers and patients for decades have faced an intimidatingly confusing mix of sometimes contradictory information regarding the true effects of cannabis on the brain and, specifically, memory and cognition.
Since the early 20th century, prohibitionists have been claiming that cannabis use by humans produces a variety of mental problems and deficiencies, from killing brain cells to a range of psychiatric disorders to the lowering of intelligence.
Programs like D.A.R.E (launched in 1983 in, ironically, Los Angeles) preached the dangers of marijuana use to school children throughout the United States. The natural herb was included with hard drugs known to involve serious physical addiction and sometimes fatal withdrawal symptoms, including heroin and cocaine. Such efforts have served to taint the reputation of cannabis and convince millions of consumers that prolonged use of the plant may result in problems such as memory deficiencies or other cognitive problems.
Meanwhile, scientists for decades have touted the potential of phytomolecules such as delta-9 tetrahydrocannabinol (THC) and cannabidiol (CBD) to do things such as improve the neuroplasticity and overall health of brain cells. This is of understandably significant consequence to the large patient populations afflicted with conditions such as Alzheimer's, Parkinson’s, and autism.
What Six Research Studies Reveal
Does Cannabis Affect Memory? With such conflicting evidence promoted to several generations of North Americans, what does the hard science and research say? Below are six peer-reviewed research studies conducted over the course of the past two decades regarding the topic of the effect of long-term cannabis use on memory function and overall cognitive performance.
2021 Study
A 2001 study entitled "Neuropsychological Performance in Long-term Cannabis Users" that was published in the journal Archives of General Psychiatry investigated the longitudinal impact of cannabis use on cognition.
The human clinical trial study involved 180 test subjects categorized as 63 “current heavy users" (who had consumed at least 5000 times during their lives and were daily users at the time of the study), 45 "former heavy users" (who had consumed at least 5000 times during their lives but fewer than a dozen times in the past three months), and 72 control subjects who had consumed fewer than 50 times during their lives.
The human clinical trial study involved 180 test subjects categorized as 63 “current heavy users" (who had consumed at least 5000 times during their lives and were daily users at the time of the study)
All participants abstained from cannabis consumption for 28 days (confirmed by urinalysis), a period over which cognitive and memory performance data was gathered (at day 0, 1, 7, and 28). Reported the study’s authors, "We administered a neuropsychological test battery to assess general intellectual function, abstraction ability, sustained attention, verbal fluency, and ability to learn and recall new verbal and visuospatial information."
The current heavy users group "scored significantly below control subjects on recall of word lists" during the performance tests administered on days 0, 1, and 7. The researchers found this deficit to be directly and proportionally associated with the level of THC in the blood of test subjects when the study was launched.
"By day 28, however, there were virtually no significant differences among the groups on any of the test results," reported the pioneering study. The researchers concluded that their data could detect "no significant associations between cumulative lifetime cannabis use and test scores."
2005 Study
A 2005 study entitled "Neurocognitive Consequences of Marihuana—a Comparison with Pre-drug Performance" that was published in the journal Neurotoxicology and Teratology explored the "effects of current and past regular use of marihuana [sic]."
The study involved examination of 113 young adults "evaluated using neurocognitive tests for which commensurate measures were obtained prior to the initiation of marihuana smoking." The researchers assessed "overall IQ, memory, processing speed, vocabulary, attention, and abstract reasoning." Like other studies, data gathered indicated that "heavy [cannabis] users did significantly worse than non-users in overall IQ, processing speed, and immediate and delayed memory."
"By day 28, however, there were virtually no significant differences among the groups on any of the test results," reported the pioneering study. The researchers concluded that their data could detect "no significant associations between cumulative lifetime cannabis use and test scores."
However, like the 2001 study cited above, "the former marihuana smokers did not show any cognitive impairments." The study’s authors concluded that cognitive deficiencies "are evident beyond the acute intoxication period in current heavy users...but similar deficits are no longer apparent three months after cessation of regular use." The scientists noted that this was true “even among former heavy using young adults."
2006 Human Trial Study
A 2006 human trial study entitled "Long-term Effects of Frequent Cannabis Use on Working Memory and Attention" that was published in the journal Psychopharmacology investigated “brain function in frequent but relatively moderate cannabis users in the domains of working memory and selective attention."
The study involved a relatively small group of only ten human participants who "performed equally well during the working memory task and the selective attention task." The researchers reported that cannabis users "did not differ from controls in terms of overall patterns of brain activity in the regions involved in these cognitive functions." However, the report did note that "in comparison to the controls, cannabis users displayed a significant alteration in brain activity in the left superior parietal cortex."
Despite its low number of test subjects, this research echoes others that indicate little or no difference between consumers and non-consumers of cannabis in terms of memory and cognitive performance—especially after a sustained period of cessation (typically seven to 90 days).
2014 Study
A 2014 study entitled "Effect of Baseline Cannabis Use and Working-Memory Function on Changes in Cannabis Use in Heavy Users" that was published in the journal Human Brain Mapping intended to "assess the predictive power of working-memory function for future cannabis use and cannabis-related problem severity in heavy users."
The researchers reported that "behavioral performance and working-memory function did not significantly differ between heavy cannabis users and control [participants]."
The human trial study involved 73 participants aged 18-25, including 32 "heavy cannabis users" (defined as using more than 10 days per month) and 41 "nonusing control" participants who had consumed fewer than 50 cannabis joints during their life and had not used during the year immediately prior to study participation.
The researchers reported that "behavioral performance and working-memory function did not significantly differ between heavy cannabis users and control [participants]."
2018 Study
A 2018 study entitled "One Month of Cannabis Abstinence in Adolescents and Young Adults is Associated with Improved Memory" that was published in The Journal of Clinical Psychiatry explored "associations between adolescent cannabis use and poor neurocognitive functioning." The study involved 88 test subjects aged 16-25 "who used cannabis regularly."
The researchers concluded that their data "suggests that cannabis abstinence is associated with improvements in verbal learning that appear to occur largely in the first week following last use." These results supported the observations of the 2001 study cited above that found cognitive deficiencies in heavy users only during the first week of abstinence, but no such difference from the control group after 28 days.
2018 Study
A 2018 study entitled "Biphasic Effects of THC in Memory and Cognition" that was published in the European Journal of Clinical Investigation reported that cannabis consumption results in "a reversible disruption of short-term memory induced by delta-9-tetrahydrocannabinol (THC), the primary psychoactive component of cannabis."
"THC is also able to improve neurological function in old animals when chronically administered at low dose. Compelling data have shown that memory is also affected in a biphasic fashion."
The researchers found this "reversible disruption of short-term memory" to most affect "attention, working memory, verbal learning, and memory functions." The study reported a biphasic response from THC, which produced different results depending on dosage and other usage characteristics. "THC is also able to improve neurological function in old animals when chronically administered at low dose. Compelling data have shown that memory is also affected in a biphasic fashion."
The study concluded that "THC modulates memory and cognition in a biphasic and age-dependent manner."
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