Long-Term Cannabis Use May Have Minimal Impact on Cognitive Decline, Study Finds

A new study delves into the long-term relationship between cannabis use and cognitive function, offering intriguing insights into an area that has often been overshadowed by research on short-term effects. The study, conducted on a cohort of over 5,000 Danish men, reveals a nuanced picture: while cannabis users experienced slightly less cognitive decline over four decades compared to non-users, the difference is modest and of questionable clinical significance.

Overview of the Study and Its Participants

The longitudinal research, published in Brain and Behavior in November 2024, analyzed data from the Danish Aging and Cognition (DanACo) cohort. The study followed 5,162 men born between 1949 and 1961, capturing their cognitive abilities at an average age of 20 and reassessing them at around 64. These assessments provided a rare opportunity to measure IQ changes over an average span of 44 years.

Key variables, including cannabis use frequency, age of initiation, education, lifestyle factors, and health conditions, were incorporated to provide a comprehensive view of how cannabis use intersects with cognitive aging.

Participants were divided into two main groups: cannabis users and non-users. Further categorization based on the age of first cannabis use—below 18, between 18 and 25, and older than 25—allowed researchers to investigate whether early initiation posed a higher risk for cognitive decline.

Key Findings: Cannabis Users and Non-Users Compared

The study’s findings highlight subtle yet noteworthy patterns:

• The average cognitive decline across all participants was 6.2 IQ points over 44 years.
• 39.3% of participants had used cannabis at least once, with over half initiating use before turning 18.
• Cannabis users demonstrated slightly higher average IQs in early adulthood compared to non-users. However, the IQ gap widened in favor of non-users by the follow-up assessment in late midlife.
• Adjusted models indicated that cannabis users experienced 1.3 IQ points less cognitive decline compared to non-users. Although statistically significant, researchers cautioned that this difference is unlikely to have meaningful clinical implications.

Interestingly, cannabis users were more likely to exhibit other risk factors:

• Higher rates of smoking and binge drinking.
• A greater likelihood of using other drugs, such as amphetamines.
• A higher prevalence of psychiatric disorders diagnosed in hospital settings.

Does Cannabis Use Protect Against Cognitive Decline?

The findings challenge some common assumptions about cannabis use. Contrary to fears that cannabis may accelerate cognitive decline, the study found no significant association between frequent use or age of initiation and greater cognitive decline. This result held even for those who began using cannabis before 18, a group often considered at higher risk for long-term cognitive impacts.

Researchers emphasized, however, that the observed difference in cognitive decline—1.3 IQ points—is modest at best. While it suggests that cannabis use might not be as harmful to cognitive aging as some critics claim, the protective effect is minimal and should not be overstated.

Contextual Factors and Limitations

The broader health and lifestyle profiles of cannabis users in the study present a complex picture. Higher rates of smoking, binge drinking, and psychiatric diagnoses among users suggest that other factors could confound the relationship between cannabis use and cognitive function. For example, the slightly reduced cognitive decline in cannabis users may be partially explained by selection bias, as those who used cannabis often had higher IQs to begin with.

Additionally, the study’s all-male sample limits its generalizability, and its focus on a Danish population may not fully capture diverse sociocultural and genetic influences. Future research could benefit from examining similar longitudinal trends in more diverse and mixed-gender cohorts.

Implications for Public Perception and Policy

While the study may offer reassurance to some cannabis users, it underscores the importance of measured interpretation. The modest difference in cognitive decline does not imply that cannabis use is without risks, especially given the higher prevalence of substance abuse and psychiatric disorders among users.

As cannabis legalization continues to expand globally, findings like these add depth to the discussion about its long-term effects. Policymakers and healthcare professionals might use such data to balance public health messaging, addressing potential risks while acknowledging that catastrophic cognitive decline is not an inevitable consequence of cannabis use.

This study sheds light on the complexities of cannabis use and aging. It challenges long-held stereotypes, offering a fresh perspective on how lifestyle choices influence cognition over a lifetime.