The relationship between cannabis and dementia has come under increasing scientific scrutiny as both the potency and use of cannabis have surged worldwide. With cannabis becoming more mainstream, questions have arisen about its long-term impact on cognitive health, memory, and the risk of dementia. Recent studies shed new light on these critical issues, challenging the common belief that cannabis is relatively harmless. This comprehensive blog explores the latest evidence, including major statistical studies, changes in cannabis potency, and the real implications for memory and brain health.

The Rapid Evolution of Cannabis Use and Potency

Cannabis use has transformed dramatically over the past five decades. While cannabis was once regarded as a relatively mild psychoactive, the landscape has shifted as both usage rates and the strength of available products have climbed.

Surging THC Potency

One of the most significant changes in the world of cannabis is the jump in THC (the main psychoactive compound) potency.

• 1977: The average THC concentration was 1.5%.
• 1987: This climbed to 3.65%.
• 2020s: High-potency strains like Sinsemilla regularly exceed 7%, with many extracts, oils, and modern products reaching up to 70–90% THC.

Rising Usage Rates

Global cannabis consumption has expanded alongside this increase in potency:

• United States: 29 million people currently use cannabis, with 7 million reported daily users.
• United Kingdom: 12% of males and 9% of females aged 15 to 25 use cannabis. Seizures of the drug rose from just over 6,000 kg in 1978 to 45,000 kg by 1988.
• Worldwide: According to WHO and UN estimates, there were 180.6 million users in 2011. By 2021, this had risen to 219 million.

Legalisation in multiple countries, including the US, Canada, Thailand, and the Netherlands, has contributed to both increased usage and product diversity.

Cannabis and Dementia: Beyond Short-Term Memory Loss

Cognitive impacts from cannabis go far beyond the classic “short-term memory loss” stereotype. New research points to deeper and longer-lasting issues affecting brain function, memory retention, and even overall IQ.

IQ Decline in Long-Term Cannabis Users

The renowned Dunedin Longitudinal Study in New Zealand tracked thousands of individuals over several decades, yielding powerful findings:

• Heavy Users: Those with more than 1,000 lifetime uses experienced an average IQ drop of 5.5 points from childhood to midlife.
• Non-Users: Saw a slight IQ increase of 0.7 points over the same period.
• Comparison: The IQ decline for heavy cannabis users far surpassed those for lifelong tobacco (1.5 points) or alcohol users (0.5 points).

Working Memory and Processing Speed

A recent University of Colorado study, published in JAMA Network Open, found that:

• 63% of heavy cannabis users and 68% of recent users showed reduced brain activity on tests of working memory.
• Heavy users continued to show deficits even after six weeks without cannabis, suggesting a persistent impact.
• Reduced activation was noted in critical brain areas such as the dorsolateral prefrontal cortex and anterior insula, both vital for decision-making and attention.

These deficits are not just theoretical. Impaired working memory affects real-life tasks (like holding a conversation, learning new information, or solving quick problems) and leads to slower reaction times and reduced ability to adapt to change.

Structural Brain Changes Linked to Cannabis Use

Neuroimaging studies show substantial physical changes in the brains of regular cannabis users, especially those who started young or used high-potency products:

• Reduced grey matter: Significant decreases in the hippocampus, a region central to memory formation and cognitive processing.
• The hippocampus is rich in cannabinoid receptors. This likely explains its vulnerability to damage from frequent cannabis exposure.

Persistent Deficits Even After Quitting Cannabis

Many believe the effects of cannabis on memory and cognition are reversible. Emerging evidence disputes this:

• Heavy users who quit still show ongoing memory and processing speed deficits months or even years later.
• The risk and severity are particularly high for those who started young (before age 18) or used at least four days per week.

Cannabis and Dementia Risk: Key Population Insights

One of the largest studies to date, a Canadian analysis of over six million adults, revealed a direct link between cannabis and dementia diagnosis.

Key Findings

• ER cannabis patients: Individuals who visited the emergency room for cannabis-related issues had a 23% higher risk of being diagnosed with dementia within five years, relative to non-users.
• Hospitalised cannabis users: Showed a 72% increased dementia risk within five years.
• Long-term diagnoses: 19% of studied cannabis users were diagnosed with dementia over a 10-year period, compared to 15% of non-users.
• Older Adults: There was a nearly 27-fold increase in cannabis-related medical visits among people aged 65+ between 2008 and 2021.

Known Health Risks

The findings around dementia add to established risks of chronic cannabis use:

• Mental health: Regular use is associated with higher risks for anxiety, depression, and even psychosis, especially with high-potency products.
• Heart and vascular health: Higher rates of strokes and heart attacks found in regular users.
• Respiratory issues: Especially among those who smoke cannabis.

Who Is Most at Risk? Vulnerability Factors to Watch

Early Onset

The earlier a person starts using cannabis (particularly before 18), the greater their cognitive and neurological risk. The developing adolescent brain is far more sensitive to THC and structural disruption.

Frequency and Duration

• Regular use (4+ days/week): Linked to a higher risk of permanent cognitive impairment.
• Lifetime exposure (>1,000 uses): Strong association with persistent IQ loss and memory deficits.

Potency of Products

High-THC products (including oils and extracts) are especially risky. These can contain up to 90% THC, significantly raising the risk for dependence, anxiety, psychosis, and long-term cognition issues.

Cannabis and Dementia in Clinical and Public Health Contexts

Clinical Implications

• Persistent cognitive deficits can make it harder to access treatments requiring executive function, such as cognitive–behavioural therapies.
• Mental health practitioners should screen for cannabis use in patients presenting with memory, focus, or mental health complaints, especially given the growing evidence of lasting impairment.

Public Health Messaging

The idea that cannabis is “safe” or has only mild short-term effects needs clear updating. Public health campaigns should:

• Highlight the dangers of high-potency products.
• Warn against early, frequent use.
• Address the particular vulnerabilities of adolescents and young adults.

Legalisation does not make a substance harmless. Policies should be grounded in the latest scientific evidence, not public opinion or market pressures. The growing body of research on cannabis and dementia points to a clear message. Cannabis use, particularly when started early and used heavily or at high potency, can cause lasting harm to critical thinking, memory, and brain structure. The suspected link to greater dementia risk is both scientifically and socially urgent.

Sources:

Forget Cannabis? The Marijuana Memory Minefield

New Study Links Cannabis and Dementia. Here’s What That Means

Can cannabis affect memory? Largest study yet investigates