By Meghan McCarthy
Over the past decade, cannabis legalization has dramatically shifted consumption habits across the United States. As of 2024, cannabis is legal in 39 states for medical use and 24 states for recreational use.
Estimates from 2023–2024 suggest that 15% of Americans report marijuana use—up nearly 10% from 2013. Young adults (19%) are more likely to use cannabis than adults age 55 and older (10%).
As usage increases, questions arise about how cannabis may affect brain health over a lifetime. What is its relationship to dementia risk and care? We explored recent studies to better understand both the cognitive risks of cannabis and its emerging therapeutic uses.
Understanding the Plant
Cannabis terminology can be confusing, but it’s essential to understanding its effects.
- Cannabis refers to the plant, which contains over 100 compounds called cannabinoids.
- THC (tetrahydrocannabinol) is the primary psychoactive compound—it produces euphoria and relaxation but also impairs memory and attention.
- CBD (cannabidiol) is nonpsychoactive and has shown potential for reducing inflammation, anxiety, and behavioral symptoms.
- Marijuana refers to cannabis products containing THC.
- Weed is a colloquial term for marijuana.
- Synthetic cannabinoids are lab-created versions of THC or related compounds used in controlled studies.
While THC is known for its psychoactive effects, both THC and CBD have been studied for their potential roles in brain health, sometimes protective, sometimes harmful. The effects appear to vary by dose, formulation, and purpose of use.
The Risks: Cannabis and Cognitive Function
While the relationship between cannabis and dementia risk is not fully understood, a growing body of research has investigated how cannabis affects memory, executive function, and brain structure, especially with long-term or frequent use.
A leading study by Joshua Gowin, PhD, assistant professor of Radiology at the University of Colorado School of Medicine, was among the largest to examine how cannabis affects the brain.
His team used brain imaging data from the Human Connectome Project, which includes cognitive tasks measuring working memory, social processing, pattern recognition and language. Participants were grouped by their lifetime cannabis exposure: minimal use (fewer than 10 times), intermediate, and heavy use (over 1,000 times).
“Most studies have looked at one task or type of process,” Dr. Gowin said. “We thought, wouldn’t it be nice if, within one sample, you could look at several things and better understand how cannabis affects brain function as whole.”
Key findings included:
- 68% of heavy cannabis users showed reduced brain activity during working memory tasks.
- This same was true for 68% of recent users.
- The most affected brain regions included those involved in decision-making, memory, attention, and emotional regulation.
“We expected recent use to have more impact than lifetime exposure,” he said. “But we were surprised to see such a strong effect of both.”
Dr. Gowin’s ongoing work explores whether cognitive functions can recover over time with abstinence.
These findings contribute to a broader landscape of research on cannabis and cognitive functioning:
- Working Memory: Heavy cannabis use is associated with reduced brain activity during working memory tasks.
- IQ and Learning: Long-term use correlates with IQ decline and slower processing speed.
- Executive Function: Deficits are observed in executive function, learning, memory, and decision-making.
- Older Adults: Long-term users show lower executive function and processing speed; recent use affects working memory.
- Brain Structure: Long-term use linked to smaller hippocampal volume, an area of the brain associated with memory.
The Potential: Cannabinoids in Dementia Care
On the therapeutic side, researchers are exploring how certain cannabinoids — especially in controlled medical formulations — may help alleviate neuropsychiatric symptoms in patients with Alzheimer’s disease and related dementias (ADRDs). These symptoms, such as agitation, anxiety, and appetite loss, can severely impact quality of life.
Krista Lanctôt, PhD, who leads the Geriatric Psychopharmacology Research Group at Sunnybrook Health Sciences Centre, is at the forefront of this research.
“I’ve long studied treatments for neuropsychiatric symptoms in Alzheimer’s,” Dr. Lanctôt said. “Agitation affects nearly 50% of patients in moderate to severe stages.”
In a pilot study, her team used nabilone, a synthetic THC compound delivered in capsule form (1–2 mg, twice daily). Unlike plant-based cannabis, synthetic cannabinoids allow for exact dosing and potency.
Why cannabinoids?
- They may reduce inflammation in the nervous system.
- They’ve shown anti-anxiety properties.
- They can stimulate appetite, helping patients who refuse to eat when agitated.
Results:
Participants had a significant reduction in agitation, and caregivers reported improved stress levels.
Dr. Lanctôt stresses the need for larger studies. Her team has now launched a follow-up trial using various formulations to explore broader accessibility.
“Ultimately, the goal is precision medicine,” she said. “We may one day use biomarkers and clinical factors to predict which patients will respond best. But before adding any medication, we need a large phase 3 trial to confirm efficacy.”
Other notable research includes:
- Agitation in Alzheimer’s: A randomized trial showed that dronabinol (synthetic THC) significantly reduced agitation by 30% in Alzheimer’s patients, with minimal side effects.
- Agitation in Dementia: A study using CBD-rich oil showed over 94% of patients had a 30% or more reduction in agitation; caregiver distress also decreased.
- Hospice Dementia Care: A multi-site trial, including Georgetown, is testing a THC/CBD combination to reduce end-of-life agitation in dementia patients, offering a gentler alternative to morphine.
- Behavioral Symptoms and Rigidity: A Geneva pilot study on THC/CBD therapy in patients with severe dementia showed improvement in agitation and motor rigidity.
What’s Next?
Previous therapeutic trials offered cautious optimism, with small sample sizes and limited duration. Ongoing research will provide more clarity on long-term effects, optimal dosing, and how to balance potential benefits with risks such as sedation or worsening cognitive function.
As cannabis use becomes more common, understanding its long-term impact on brain health is essential. The same plant that may offer relief for some symptoms of dementia may also carry cognitive risks when used recreationally, particularly over long periods or at high doses.
The future of cannabis in brain health lies not in blanket endorsements or fears, but in continued research focused on who benefits, who is at risk, and how to use its many compounds wisely.