Men using cannabis may actually experience less cognitive decline

COPENHAGEN, Denmark — Cannabis has had an age-old reputation of leaving its users in a foggy and cognitively impaired state. That decades-old theory may have just gone up in smoke. A new study that spanned nearly half a century has found that men who use cannabis may actually suffer from less cognitive decline as they age.

The study tracked the cognitive health of over 5,000 Danish men, challenging common assumptions about cannabis use and brain function. Spanning an impressive 44-year period, a team from the University of Copenhagen are now providing a unique window into how cannabis might — or might not — impact mental performance over a lifetime.

Cannabis has long been a controversial substance, even as more countries legalize its use. Current drug policy classifications often paint cannabis as a substance with high abuse potential and no medical value. Yet approximately 9.7% of Danish adults aged 16-44 use cannabis annually, reflecting similar usage rates across many Western nations. Given these numbers, understanding marijuana’s true long-term cognitive impacts has major public health implications.

This study set out to answer a fundamental question: Does cannabis use accelerate cognitive decline as people age?

The research published in the journal Brain and Behavior followed 5,162 men from early adulthood to late middle age, measuring their cognitive abilities through standardized intelligence tests at two key points — first when they were around 20 years-old during mandatory military enlistment, and again when they were approximately 64 years-old. The study captured an extraordinary range of life experiences, tracking participants from the late 1960s through the early 2020s.

Surprisingly, the results defied many existing assumptions. Not only did male cannabis users not show accelerated cognitive decline, but they actually demonstrated slightly less mental deterioration compared to non-users. The difference was modest — about 1.3 IQ points — but statistically significant.

Approximately 39.3% of the men in the study reported having used cannabis at least once. The researchers carefully analyzed various factors, including the age when participants first used cannabis and how frequently they used it. Contrary to some previous research suggesting that early or frequent cannabis use might harm cognitive function, this study found no significant link between the age someone starts using cannabis or frequency of use and cognitive decline.

On average, all participants experienced some cognitive decline between their 20s and 60s – about 6.2 IQ points overall. However, cannabis users declined by about 5.3 points compared to 6.8 points for non-users. This difference remained significant even after the researchers controlled for factors like education level, alcohol use, tobacco use, psychiatric history, and other drug use.

The study’s methodology was robust. Participants completed the same intelligence test at both time points, which allowed researchers to directly measure changes in cognitive performance. The test, called Børge Prien’s Prøve, assessed various mental abilities including logical reasoning, verbal skills, and numerical problem-solving.

Researchers also examined whether starting cannabis use earlier in life or using it more frequently was associated with greater cognitive decline among users. Neither factor showed significant effects. People who started using cannabis before age 18 declined cognitively at similar rates to those who started after 25, and frequent users (defined as using cannabis at least twice per week) showed similar patterns to occasional users.

Interestingly, the researchers noted that cannabis users in the study tended to have slightly higher baseline intelligence and education levels. They were also more likely to smoke tobacco and consume alcohol, suggesting that the relationship between cannabis use and cognitive function is complex and influenced by multiple lifestyle factors.

However, before cannabis advocates declare victory, the researchers emphasize several important caveats. The difference in cognitive decline between users and non-users, while statistically significant, was modest (as mentioned, about 1.3 IQ points in the fully adjusted analysis). This represents only about 7% of a standard deviation in cognitive scores, a difference that may not be meaningful in real-world terms.

Additionally, the study only included men and had a relatively low participation rate of about 14.3% of those invited to participate in the follow-up testing. The participants who did complete the study tended to have higher baseline intelligence scores and education levels than non-participants, potentially making them less representative of the general population.

The researchers also note that most of the cannabis users in the study (92.4%) had not used marijuana in the year before their follow-up testing. This means the results may better reflect the long-term effects of past cannabis use rather than ongoing use into older age.

What might explain these counterintuitive findings? The researchers point to several recent animal studies that suggest certain cannabinoids might have positive cognitive effects. For instance, studies in rats have shown cannabinoid receptor stimulation can improve memory in older animals, while research in mice has demonstrated that low doses of THC can restore cognitive function in aging subjects. However, the mechanisms behind these effects – and whether they translate to humans – remain unclear.

As cannabis policy continues to evolve worldwide, this research provides valuable long-term data about its cognitive impacts. While the findings don’t suggest that cannabis actively protects brain function, they do indicate that moderate use earlier in life may not significantly impair cognitive aging. The researchers were careful to point out that more studies are needed to fully understand its long-term impacts. Additionally, the study focused exclusively on men, so the results may not directly apply to women.