Does using cannabis earlier in life lead to faster cognitive decline as you get older? A recent study published in the journal Brain and Behavior that tracked the thinking abilities of thousands of men from young adulthood into middle age sought to answer this question. The results were unexpected: men who had used cannabis at some point in their lives did not show greater cognitive decline. Instead, they demonstrated a slightly smaller decline in cognitive function over the decades compared to men who had never used cannabis.
While we know that cannabis can have immediate effects on thinking and memory, the long-term consequences, especially in relation to age-related cognitive decline, are not well understood. Some earlier research suggested that long-term cannabis use could harm cognitive function over time, with some studies indicating a connection between regular cannabis use and steeper declines in cognitive abilities. However, other studies have not found this link, and some have even hinted at potential benefits of cannabis for cognitive function in certain contexts.
Because of these inconsistent findings and the increasing prevalence of cannabis use in society, researchers from the University of Copenhagen wanted to investigate this issue further. They aimed to determine if there was a relationship between using cannabis and the natural cognitive decline that can occur as people age from early adulthood to middle age. They also wanted to explore if factors like when someone started using cannabis or how frequently they used it made a difference in any potential link to cognitive decline.
To explore the relationship between cannabis use and cognitive decline, the researchers used data from a large, long-term study called the Danish Aging and Cognition cohort. This cohort study was designed to track factors that predict cognitive changes as people age, starting from young adulthood and continuing into later middle age. The data came from combining two similar follow-up studies.
The basis of these studies was the conscription board intelligence test, which is given to most young men in Denmark when they are between 18 and 26 years old as part of the military conscription process. The researchers re-examined the cognitive abilities of these men in their late middle age, on average 44 years after their initial conscription test. At the follow-up examination, the men also completed a detailed questionnaire about their lives, including socioeconomic factors, lifestyle choices, and health.
The study started with over 5,300 men who participated in the follow-up studies. However, due to some technical issues and missing information about cannabis use, the final study group included 5,162 men. These men were born between 1949 and 1961. Their initial intelligence tests were conducted between 1967 and 1989 when they were around 20 years old on average. The follow-up examinations took place between 2015 and 2022, when the men were around 64 years old on average.
To understand cannabis use, the researchers asked the men at the follow-up examination about their past and present use of illegal drugs in Denmark. For this particular study, they focused on cannabis use. They categorized men into two groups: those who had ever used cannabis and those who had never or almost never used cannabis. For those who had used cannabis, they also gathered information about when they started using it and how frequently they used it.
The age of starting cannabis use was grouped into three categories: before age 18, between 18 and 25, and after age 25. Information on how often they used cannabis was collected in one part of the study. Frequent cannabis use was defined as using cannabis a couple of times a week or more. The researchers also estimated the number of years each man had used cannabis frequently by looking at their reported use across different age periods.
Cognitive ability was measured using the conscription board intelligence test, called the Børge Priens Prøve. This test is known to be reliable and is similar to standard intelligence tests. It includes different types of questions, such as those involving letter patterns, word analogies, number sequences, and geometric shapes. The researchers used the scores from this test, taken both at conscription in early adulthood and at the follow-up in late middle age, to calculate cognitive decline. Cognitive decline was defined as the difference in intelligence test scores between the two time points.
In addition to cannabis use, the researchers also collected information on other factors that could influence cognitive decline. These included the length of time between the two intelligence tests, education level, alcohol consumption patterns (specifically, extreme binge drinking), smoking habits, use of other illegal drugs, any history of psychiatric disorders, and general physical health conditions, measured using a standard index called the Charlson Comorbidity Index.
On average, the men in the study experienced a cognitive decline of about 6 IQ points over the 44-year period. Interestingly, men who had used cannabis at some point in their lives actually showed slightly less cognitive decline compared to men who had never used cannabis. This finding held even after considering other factors like age, education, lifestyle, and health.
Specifically, in their most comprehensive analysis, cannabis users showed about 1.3 IQ points less decline than non-users. While this difference was statistically noticeable, the researchers noted that the size of this difference was small and might not be practically meaningful in everyday life.
When the researchers looked at different aspects of cannabis use among users, such as the age when they started using cannabis or how many years they used it frequently, they did not find any significant link to cognitive decline. Whether someone started using cannabis before age 18, between 18 and 25, or after 25 did not seem to affect their cognitive decline in late middle age. Similarly, the number of years of frequent cannabis use was not related to a greater or lesser degree of cognitive decline.
The researchers acknowledged some limitations of their study. One important limitation is the low participation rate in the follow-up study. Only a small percentage of the men who were initially invited actually participated in the follow-up examinations. This could mean that the men who participated are not fully representative of the original group, potentially affecting how broadly the findings can be applied. It is possible that men with certain characteristics, such as heavier cannabis use or greater cognitive decline, might have been less likely to participate, although it is not clear if participation would be directly related to cognitive decline based on cannabis use.
Another limitation is that the information on cannabis use was based on what the men reported themselves at the follow-up examination, which could be subject to recall errors or underreporting due to social desirability. Additionally, the study only included men, so the findings might not be the same for women, as cannabis could affect men and women differently.
Despite these limitations, the study has notable strengths, including the long follow-up period, the use of the same intelligence test at both time points, and the availability of detailed information on various factors that could influence cognitive decline. The long time between the initial and follow-up tests also reduces the impact of practice effects on the cognitive measurements.
Future research should investigate why this study found slightly less cognitive decline in cannabis users. It is possible that this finding is not directly due to cannabis itself, but rather reflects other differences between cannabis users and non-users that were not fully accounted for in the study. It is also important for future studies to include women and to examine the effects of cannabis use across different age ranges and patterns of use in more detail. Further research could also explore whether any cognitive effects of cannabis use are reversible over time if use is discontinued.
The study, “Cannabis Use and Age-Related Changes in Cognitive Function From Early Adulthood to Late Midlife in 5162 Danish Men,” was authored by Kirstine Maarup Høeg, Rasmus Ljungbeck Frodegaard, Marie Grønkjær, Merete Osler, Erik Lykke Mortensen, Trine Flensborg-Madsen, and Gunhild Tidemann Okholm.
