What is long-term cannabis use doing to our brains?

By Joe Hannan | Medically reviewed by Amanda Zeglis, DO, MBA
Published August 17, 2022

Key Takeaways

  • A 2022 study found that long-term cannabis users had cognitive deficits and lower hippocampal volume at age 45 compared with non-users and users of other substances.

  • Legalization and decriminalization efforts, coupled with medical marijuana programs, may be making midlife cannabis use more common.

  • Clinicians may want to consider making cannabis-use screening a mainstay of clinical practice for middle-aged patients to avoid potential memory declines.

Is cannabis as harmless as some would think? A March 2022 study published in the American Journal of Psychiatry suggests otherwise for middle-aged adults.[]

Research has already indicated that cannabis use is detrimental to cognitive development in adolescents.

A 2020 Frontiers in Psychiatry study found that the adolescent mind is susceptible to the neurocognitive effects of cannabis, and these effects may linger into adulthood.[]

What’s remained somewhat mysterious is how cannabis use affects the brain in midlife. This information is crucial as legalization and decriminalization efforts spread across the US and more middle-aged people begin to partake.

Research sheds light on effects

Currently there are only 11 states without medical or recreational cannabis programs.[] And according to a 2020 JAMA Internal Medicine research letter, cannabis use among Americans aged 65 years and older rose from .4% in 2006 and 2007 to 2.9% in 2015 and 2016.[]

Madeline H. Meier, Ph.D., an associate professor in the department of psychology at Arizona State University and one of the lead researchers of the American Journal of Psychiatry study, is working to shed light on how cannabis affects this growing group of users.

In an MDLinx interview, Dr. Meier said her team’s study reveals important insights about how cannabis affects adults.

"What we found was that the long-term cannabis users did show midlife cognitive deficits relative to non-users and relative to long-term tobacco and alcohol users."

Madeline H. Meier, Ph.D.

“I think that's pretty important, because it distinguishes whether cannabis is more or less harmful than other substances, which is a point made by proponents of legalization,” Dr. Meier added.

This longitudinal study involved a group of 1,037 adults in New Zealand, born between 1972–1973 and followed until age 45. Ninety-four percent of the participants completed the study. Researchers assessed cannabis use and dependence at ages 18, 21, 26, 32, 38, and 45. They also documented IQ at ages 7, 9, 11, and 45.

Researchers measured neurophysiological functions and hippocampal volume at age 45. Those functions included learning, memory, executive functioning, processing speed, verbal comprehension, perceptual reasoning, and motor function. These abilities, plus hippocampal volume, can all be indicative of dementia risk, underscoring the need to identify how cannabis affects this population segment.

Study reveals IQ decline

Those who consistently used cannabis had a mean IQ decline of 5.5 points at midlife.

They also exhibited informant-reported memory and attention problems, as well as poorer learning and processing speed relative to childhood IQ levels.

Long-term cannabis users also had smaller hippocampal volumes. Hippocampal volume was of interest to Meier and her team because of its link to dementia, its involvement in learning and memory, and its concentration of cannabinoid receptors.

While the study clearly showed lower hippocampal volumes in long-term cannabis users, the lower volume did not explain the differences between long-term users and non-users, Meier said.

What the study did reveal, however, was some striking comparisons between tobacco and alcohol use.

Comparing substances

Dr. Meier and her team compared findings for long-term cannabis users with those for participants who didn’t use cannabis, long-term tobacco users, long-term drinkers, midlife recreational cannabis users, and cannabis quitters. Adjusted mean scores for 10 neurophysiological functions were in many cases worse for cannabis users than for tobacco users and drinkers.

"I think the group that surprised me in particular was the long-term alcohol users."

Madeline H. Meier, Ph.D.

“(Long-term cannabis users) are even performing worse in some cases," Dr. Meier continued. "That’s a little surprising, given what we know about the association between long-term cannabis use or long-term alcohol use and cognitive functioning. I was a little surprised that the long-term alcohol users weren’t performing worse there as well.”

This finding, and the study’s other findings, have some immediate clinical applications, Dr. Meier said.

Putting it into practice

First, clinicians should be aware that more of their patients—especially older adults—are using cannabis, highlighting the need to stay abreast of research on cannabis use.

"Our study shows that it's long-term use that is associated with risk for midlife cognitive deficits, whereas short-term and infrequent use is not."

Madeline H. Meier, Ph.D.

“I think that's important for physicians to know,” Dr. Meier added.

Midlife or older patients who are complaining about attention and memory difficulties should be asked about their cannabis use. And if they think the two aren’t related, Dr. Meier suggested that “physicians should encourage them to kind of rethink that. Encourage them to think about how maybe quitting could benefit [their] cognitive function and set [them] up better for later life.”

What this means for you

Cannabis use doesn’t just affect developing adolescent minds. Long-term use is associated with deficits in neurophysiological function and lower hippocampal volume at midlife. These findings highlight the potential need to make cannabis use a more formal component of substance-use assessment, especially in older adults.

Read Next: What doctors should know about the latest cannabis trends
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