A new study of more than 5,000 men whom researchers evaluated over the course of 44 years found “no significant harmful effects of cannabis use on age-related cognitive decline.” In fact, the report says, “Men with a history of cannabis use had less cognitive decline from early adulthood to midlife compared to men without a history of cannabis use.”
“Among cannabis users,” the study adds, “neither age of initiation of cannabis use nor frequent use was significantly associated with greater age-related cognitive decline.”
Authors of the research, which was published this month in the journal Brain and Behavior, said that while past studies have shown evidence of short-term negative effects of cannabis use on cognition, “only a limited number of studies have explored the association between cannabis use and age-related cognitive decline.”
“This study contributes to the sparse knowledge on this subject and aligns with most existing studies, suggesting no association between cannabis use and greater cognitive decline,” wrote the seven-person University of Copenhagen research team. “More specifically, in the present study, cannabis users experienced slightly less cognitive decline compared to nonusers, and the association remained significant when controlling for potential confounders.”
Over the 44-year study period, which measured subjects IQs in early adulthood and then again later in life, the mean cognitive decline was 6.2 IQ points among all the 5,162 Danish men who participated. Marijuana users, however, showed a smaller decline.
“Notably, cannabis users exhibited statistically significantly less cognitive decline compared to nonusers,” the report says. “In the fully adjusted model, cannabis use was associated with 1.3 IQ points less cognitive decline than the decline observed in the reference group.”
Even long-term, frequent cannabis use didn’t appear to contribute to cognitive decline. “Years of frequent cannabis use were generally associated with no significant difference in cognitive decline when compared with no frequent use,” the study says.
Authors acknowledged that the difference in cognitive decline between cannabis users and nonusers may not “hold clinical significance,” as the lesser cognitive decline among users was “modest” and represented just seven percent of one standard deviation. Still, the findings align with past research that similarly found no differences in cognitive decline between cannabis users and nonusers.
Paul Armentano, deputy director of the cannabis reform group NORML, said the study’s findings “contradict one of the more prominent and longstanding stereotypes about cannabis and cannabis consumers.”
“It is unfortunate that these stereotypes often go unchallenged in the media and elsewhere,” Armentano said. “It is even more unfortunate that studies refuting these long-held stereotypes seldom receive the type of mainstream attention they deserve.”
Interestingly, authors of the new Danish study note that the “observed association of less cognitive decline among cannabis users compared to nonusers in this study may reflect characteristics of cannabis users rather than the direct effects of cannabis itself.”
“For example, cannabis users tended to have higher baseline IQ and education levels, and they tended to smoke more tobacco and consume more alcohol,” the team wrote. “Hence, it is reasonable to assume that additional unmeasured factors might influence the association, potentially confounding the result. Nevertheless, our findings of less cognitive decline among cannabis users compared to nonusers align with previous in vivo studies indicating that cannabinoids have a positive impact on cognitive function and memory in rats and mice.”
Separate federally funded research published earlier this year by the American Medical Association found that cognitive attributes such as working memory, reward and inhibitory control were not significantly affected after a year of cannabis consumption.
“Our results suggest that adults who use cannabis, generally with light to moderate use patterns, for symptoms of pain, anxiety, depression, or poor sleep, experience few significant long-term neural associations in these areas of cognition,” concluded that study, funded by the National Institute on Drug Abuse (NIDA) and published in the journal JAMA Network Open in September.
Another study found that low doses of delta-9 THC had an “antiaging effect on the brain by restoring new cognitive abilities and synapse densities in old mice,” noting that the findings “could be the basis for an effective antiaging and pro-cognitive medication.”
Yet another report on subjective cognitive decline (SCD), published earlier this year in the journal Current Alzheimer Research, found that people who consumed cannabis for recreational or medical purposes reported less confusion and memory loss compared to nonusers.
The study—which showed that recreational cannabis use is “significantly” linked to lower SCD—is especially notable given that past research has connected subjective decline to the development of dementia later in life.
Late last year, separate research indicated that “prescribed medical cannabis may have minimal acute impact on cognitive function among patients with chronic health conditions.”
“The absence of evidence for cognitive impairment following medical cannabis self-administration was surprising,” that study said, “given prior and substantive evidence that non-medical (‘recreational’) cannabis use reliably impairs a range of cognitive functions. At the same time, these findings are consistent with two systematic reviews published in the last year that suggest that medical cannabis, when used regularly and consistently for a chronic health concern, may have little if any impact on cognitive function.”
While the long-term effects of cannabis use are far from settled science, findings from a number of recent studies suggest some fears have been overblown.
A report published last year that drew on dispensary data, for instance, found that cancer patients reported being able to think more clearly when using medical marijuana. They also said it helped manage pain.
A separate study of teens and young adults at risk of developing psychotic disorders found that regular marijuana use over a two-year period did not trigger early onset of psychosis symptoms—contrary to the claims of prohibitionists who argue that cannabis causes mental illness. In fact, it was associated with modest improvements in cognitive functioning and reduced use of other medications.
“CHR youth who continuously used cannabis had higher neurocognition and social functioning over time, and decreased medication usage, relative to non-users,” authors of that study wrote. “Surprisingly, clinical symptoms improved over time despite the medication decreases.”
A separate study published by the American Medical Association (AMA) that looked at data from more than 63 million health insurance beneficiaries found that there’s “no statistically significant increase” in psychosis-related diagnoses in states that have legalized marijuana compared to those that continue to criminalize cannabis.
Studies from 2018, meanwhile, found that marijuana may actually increase working memory and that cannabis use doesn’t actually change the structure of the brain.
And, contrary to then-President Trump’s claim that marijuana makes people “lose IQ points,” the National Institute of Drug Abuse (NIDA) says the results of two longitudinal studies “did not support a causal relationship between marijuana use and IQ loss.”
Research has shown that people who use cannabis can see declines in verbal ability and general knowledge but that “those who would use in the future already had lower scores on these measures than those who would not use in the future, and no predictable difference was found between twins when one used marijuana and one did not.”
“This suggests that observed IQ declines, at least across adolescence, may be caused by shared familial factors (e.g., genetics, family environment), not by marijuana use itself,” NIDA concluded.
