The Connection Between Cannabis And Memory: What Science Tells Us

This article was originally published on The Cannabis Community and appears here with permission.

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.

Research Study Results

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.

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.

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.”

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.”

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.”