The idea that cannabis damages the brain in the long run has long been taken for granted. A Danish study that followed more than 5,000 men for over four decades paints a very different picture. The men who had used cannabis did not lose more cognitive ability with age than those who never had. If anything, the opposite was true.
Update 2026: Since this article was first written, more studies have pointed in the same direction. In February 2026, researchers at the University of Colorado published a study of more than 26,000 middle-aged and older adults in the UK Biobank. It found that lifetime cannabis use was linked to larger volume in certain brain regions and better results on several cognitive tests. More on this further down.
At a glance
- The study followed 5,162 Danish men for an average of 44 years.
- The men were tested at military conscription, around age 20, and again at an average age of 64.
- Those who had used cannabis showed 1.3 IQ points less decline than those who never had.
- Neither early initiation nor regular use was linked to greater decline.
- The study was published in the journal Brain and Behavior in 2024.
The study that turns old assumptions upside down
Researchers at the University of Copenhagen and the Capital Region of Denmark examined the link between cannabis use and cognitive change from early adulthood to late midlife. A total of 5,162 Danish men took part, and the average follow-up time was 44 years. That makes it one of the longest studies of its kind.
The results were published in 2024 in the scientific journal Brain and Behavior and go against many earlier assumptions about cannabis and the brain.
Less cognitive decline among cannabis users
Everyone loses some cognitive ability with age. In the study, scores fell by just over 6 IQ points on average over the 44 years. But the men who had used cannabis showed less decline than those who never had. The difference corresponded to 1.3 IQ points.
The result held even after the researchers accounted for factors such as education, alcohol, smoking and other drugs. Their conclusion was that they could see no harmful effects of cannabis on age-related cognitive decline in this group.
No effect from age of first use or frequency
The researchers also looked at whether the age at which people started, or how often they used cannabis, made any difference. The results showed that:
- early initiation, before the age of 18, was not linked to greater decline
- regular use, at least twice a week, was not linked to greater decline either
- there were no clear differences between different groups of users
Around 39 percent of the participants had used cannabis at least once, and roughly half of them had started before the age of 18.
How the study was done
The study was based on Børge Priens Prøve, the intelligence test Danish men take at military conscription, usually around the age of 20. The same test was repeated at follow-up, when the men were 64 on average.
This gave the researchers a rare opportunity to measure real change in the same person over an entire adult life, rather than comparing different people at a single point in time. Participants also answered questions about their cannabis use over the years, such as when they started, how often they used cannabis and whether they had tried other drugs.
What could explain the results?
The researchers point to several possible explanations. The cannabis users in the study had a slightly higher IQ and level of education from the start. The researchers adjusted for this, but there may be other factors that could not be measured.
The results are also in line with animal research. A widely cited study in Nature Medicine in 2017 showed that low doses of THC improved memory and learning in old mice. This has raised the question of whether certain compounds in cannabis could matter for the ageing brain, although it is too early to draw conclusions about humans.
More studies point the same way
The Danish study is not alone. As early as 1999, American researchers followed 1,318 people in Baltimore for 12 years and found no difference in cognitive decline between heavy users, light users and people who did not use cannabis.
A New Zealand study from 2012 did find a link between long-term use starting in adolescence and falling IQ. But the following year, a Norwegian researcher showed in the same journal that the result could be explained by socioeconomic differences between the groups.
In recent years, even more studies have been published:
- 2024: An American study of 4,744 people over 45 found that those who used cannabis for non-medical reasons were considerably less likely to feel that their memory and thinking had declined.
- 2026: The University of Colorado study of 26,362 people aged 40 to 77 found that lifetime cannabis use was linked to larger volume in brain regions rich in CB1 receptors and better results on tests of learning, memory, processing speed and attention. Moderate use showed the most favourable results.
If you want to know more about how cannabinoids interact with the body, you can read about the endocannabinoid system. We also explain the difference between CBD and THC.
Good to keep in mind
Like all research, these studies have their limitations. The Danish study only included men, and the results may differ for women. Information on cannabis use was self-reported, and people with very heavy use may have been less likely to take part in the follow-up.
The researchers also note that a difference of 1.3 IQ points is small and may not be noticeable in everyday life. The key message is rather that the study does not show the clear decline many have feared. This applies to long-term effects. It is well known that cannabis affects memory and concentration while a person is under its influence.
Conclusion
The Danish study is an important contribution to our knowledge about cannabis and the brain. With 44 years of follow-up and more than 5,000 participants it carries real weight, and it is supported by several newer studies from both the US and the UK.
Taken together, the research suggests that fears of cannabis impairing thinking in the long run have been exaggerated. At the same time, the results are based on associations rather than proven causes. The brain keeps developing until around the age of 25, and research on young people specifically is still more mixed.
What did the Danish study show?
Men who had used cannabis showed 1.3 IQ points less cognitive decline between the ages of 20 and 64 than men who had never used cannabis.
How many people took part in the study?
The study included 5,162 Danish men who were followed for an average of 44 years.
Did the age of first use matter?
No. Neither starting before the age of 18 nor using at least twice a week was linked to greater decline.
Are there other studies with similar results?
Yes. An American study from 1999 found no difference in cognitive decline, and a British study from 2026 linked cannabis use to better results on cognitive tests.
Is this medical advice?
No. The article summarises research and is intended as general information.
This article summarises scientific research and is intended as general information only. It is not medical advice. Please consult a doctor or other healthcare professional if you have questions about your health.
Sources
- Høeg KM, Frodegaard RL, Grønkjær M, Osler M, Mortensen EL, Flensborg-Madsen T, Okholm GT. (2024). Cannabis Use and Age-Related Changes in Cognitive Function From Early Adulthood to Late Midlife in 5162 Danish Men. Brain and Behavior, 14(11), e70136.
- Guha A et al. (2026). Lifetime Cannabis Use Is Associated with Brain Volume and Cognitive Function in Middle-Aged and Older Adults. Journal of Studies on Alcohol and Drugs, 87(4), 739-751.
- Chen Z, Wong R. (2024). Association Between Cannabis Use and Subjective Cognitive Decline: Findings from the Behavioral Risk Factor Surveillance System. Current Alzheimer Research, 20(11), 802-810.
- Lyketsos CG, Garrett E, Liang KY, Anthony JC. (1999). Cannabis use and cognitive decline in persons under 65 years of age. American Journal of Epidemiology, 149(9), 794-800.
- Meier MH et al. (2012). Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS, 109(40), E2657-E2664.
- Rogeberg O. (2013). Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status. PNAS, 110(11), 4251-4254.
- Bilkei-Gorzo A et al. (2017). A chronic low dose of Δ9-tetrahydrocannabinol (THC) restores cognitive function in old mice. Nature Medicine, 23(6), 782-787.
