Study Finds Cannabis Use Doesn’t Cause Later IQ Decline

Study Finds No Link Between Lifetime Cannabis Use and Cognitive Decline

Longstanding assumptions that cannabis use contributes to cognitive decline are being challenged by new research. A recent longitudinal study, published in the journal Brain and Behavior, found no association between lifetime cannabis use and age-related cognitive decline. These findings add to a growing body of evidence refuting stereotypes about cannabis users and cognitive health.

The Study: Investigating Long-Term Cannabis Use

Danish researchers analyzed data from a cohort of 5,162 men over a 44-year period. Participants’ IQ levels were assessed in early adulthood (average age: 22) and re-evaluated in late midlife (average age: 62). The study specifically examined the relationship between cannabis use and changes in cognitive performance over time.

Surprisingly, participants who used cannabis exhibited “significantly less cognitive decline” compared to those who had never consumed cannabis.

Key Findings on Cannabis and Cognition

The study revealed several critical insights:

Reduced Cognitive Decline Among Cannabis Users:
On average, participants experienced a decline of 6.2 IQ points over the 44-year study period. Cannabis users, however, demonstrated a slower rate of cognitive decline, losing 1.3 fewer IQ points than non-users.

No Impact from Age of Initiation or Frequency of Use:
Researchers found no significant differences in cognitive outcomes based on the age participants began using cannabis or the frequency of their use.

Confounder-Adjusted Models Support Findings:
Even after adjusting for potential confounding variables, the association between cannabis use and reduced cognitive decline remained statistically significant.

Authors’ Conclusions: A Call for More Research

The authors emphasized that their findings align with most existing studies, which have also found no significant links between cannabis use and greater cognitive decline. They highlighted the need for further research to determine whether these results reflect a genuine absence of adverse effects or if any impacts of cannabis use are temporary and diminish over time.

Challenging Stereotypes About Cannabis Users

The study’s findings challenge persistent stereotypes about cannabis users. Critics of cannabis reform often cite concerns about cognitive impairment, but these results suggest that such claims may be overstated or unfounded.

NORML Deputy Director Paul Armentano commented on the study, calling for greater attention to research that dispels myths surrounding cannabis and cognitive health. Despite the growing evidence, such findings often fail to gain widespread media coverage.

Historical Context: Cannabis and Cognitive Decline

While the debate about cannabis’s impact on IQ and cognition is not new, few longitudinal studies have supported the claim that cannabis use leads to cognitive decline. Key examples include:

British Study of Adolescents:
Research involving over 2,000 teenagers found that cannabis exposure before age 15 did not predict lower IQ scores or poorer educational outcomes after accounting for socioeconomic and other confounders.

Adolescent Twin Studies:
Studies involving twins have similarly failed to identify causal links between cannabis use and declines in IQ or cognitive performance.

Literature Review in JAMA Psychiatry:
A comprehensive review concluded that the cognitive effects of cannabis use in adolescents and young adults are minor and often reversible with abstinence.

Revisiting Flawed Claims

A widely publicized 2012 study suggested a link between early cannabis use and lower IQ at midlife. However, this research faced criticism for failing to properly control for socioeconomic factors. Critics argued that the study’s conclusions were likely overestimates and that the true impact of cannabis use on IQ could be negligible or nonexistent.

Subsequent responses and studies, including one published in the Proceedings of the National Academy of Sciences, further questioned the methodology and findings of the 2012 paper.

Recent Data on Cannabis and Brain Health

More recent research continues to affirm the absence of significant cognitive harm from cannabis use. For instance, a 2024 study published in the Journal of the American Medical Association reported that adult medical cannabis users did not experience adverse changes in brain structure or cognitive performance over time.

Implications for Public Perception and Policy

The findings of the Danish study carry implications for both public health narratives and cannabis policy reform:

Destigmatizing Cannabis Use:
Research challenging the link between cannabis and cognitive decline could help dispel negative stereotypes about cannabis users.

Informing Policy Decisions:
Evidence-based policymaking requires accurate information about cannabis’s risks and benefits. Studies like this could influence debates around legalization and medical cannabis use.

Encouraging Further Research:
Longitudinal studies provide valuable insights, but more research is needed to fully understand the long-term effects of cannabis use across diverse populations.

A Shift in the Narrative

The Danish study adds to a growing body of evidence suggesting that lifetime cannabis use does not contribute to significant cognitive decline. These findings challenge longstanding myths and underscore the importance of basing public health discussions on rigorous scientific evidence.

As research continues to evolve, it remains crucial to separate fact from fiction in the ongoing conversation about cannabis’s role in society.