What Happens To Your Brain When You Use Cannabis For Years?

This article was originally published in March 2023.

What happens to our brain when we smoke marijuana for years?

A study published in the Journal of American Psychiatry titled "Long-term Cannabis Users Show Lower Cognitive Reserves and Smaller Hippocampal Volume in Midlife" warned that the effects of marijuana use could be damaging.

The findings were recently revisited by San Francisco-based addictions psychiatrist, Timmen L. Cermak, M.D., who highlighted the potential dangers of long-term cannabis use, summarizing the research, co-authored by Madeline H. Meier, Ph.D.

First things first, what is the hippocampus? 

Plasticity And Vulnerability 

The hippocampus, a complex brain structure embedded deep into the temporal lobe, plays a key role in learning and memory. Cermak explained how studies on London taxi drivers confirmed its plasticity: the more the drivers memorized streets and alleys in the city, the larger their hippocampus grew. 

On the other hand, the hippocampus is also vulnerable, writes Cermak, referring to studies showing cannabis use leads to making it smaller. In addition, other researchers have pointed to the connection between decreased hippocampal volume and cognitive decline. 

Another recent study, published in the journal Neuropsychopharmacology looked at cannabis use on the hippocampus in animals. Researchers from the Schulich School of Medicine & Dentistry analyzed the dorsal and ventral regions of the brain following THC exposure in rats and concluded that it may induce long-lasting memory impairment and increased anxiety levels. Using a rodent model, scientists identified various abnormalities in each brain region. 

The study discovered several key molecular and neuronal anomalies in certain regions of the hippocampus that could be the cause of cognitive and emotional side effects. The study's authors raised concerns about increased teenage marijuana consumption.

Meier, et.al.’s study, conducted on humans is in line with these findings.

Study Highlights 

Researchers studied around a thousand individuals born in Dunedin, New Zealand, in 1972 and 1973 into their fifth decade. The participants' baseline cognitive abilities were measured at age 13 (before any drug consumption) and each was questioned about substance use at 18,21, 26, 32, 38, and now 45. IQ levels were measured at 7,9,11 and 45, while particular neuropsychological functions and hippocampal volume were assessed at 45. 

Relying on neurocognitive testing at 38, the scientists discovered that those participants who started marijuana use in adolescence and continued through adulthood had an average eight-point IQ decline since age 14. 

“These deficits were specific to long-term cannabis users because they were either not present or were smaller among long-term tobacco users, long-term alcohol users, midlife recreational cannabis users, and cannabis quitters,” the authors wrote. “Cognitive deficits among long-term cannabis users could not be explained by persistent tobacco, alcohol, or other illicit drug use, childhood socioeconomic status, low childhood self-control, or family history of substance dependence." 

A short summary includes the following findings: 

• Long-term marijuana users (usually 1-4 times weekly) at the age of 45 had a mean 5.5 points IQ decline (compared to childhood), and poorer learning and processing abilities. 
• The negative impact of marijuana depends on the dosage and frequency of use – recreational cannabis use (less than weekly) in midlife has not been confirmed to cause notable effects on cognitive capabilities (their IQ declined a mean 3.5 points.) 
• The negative effects on cognitive function don’t equal dementia, at least not at the age of 45. 

Concluding Thoughts

Considering that previous studies have shown that mild cognitive deficits and larger hippocampal atrophy in midlife increases the chances of dementia after age 70, it is important to continue to research.

Studies should continue for some 30 years for participants of this study to reach the age most common for dementia symptoms to develop so that the connection between long-term cannabis use and dementia can be confirmed.

Photo: Benzinga edit with images by hainguyenrp, lindsayfox on Pixabay