Heavy marijuana users have abnormal brain structure and poor memory

Teens who were heavy marijuana users - smoking it daily for about three years - had abnormal changes in their brain structures related to working memory and performed poorly on memory tasks, reports a new Northwestern Medicine® study.

A poor working memory predicts poor academic performance and everyday functioning.

The brain abnormalities and memory problems were observed during the individuals' early twenties, two years after they stopped smoking marijuana, which could indicate the long-term effects of chronic use. Memory-related structures in their brains appeared to shrink and collapse inward, possibly reflecting a decrease in neurons.

The study also shows the marijuana-related brain abnormalities are correlated with a poor working memory performance and look similar to schizophrenia-related brain abnormalities. Over the past decade, Northwestern scientists, along with scientists at other institutions, have shown that changes in brain structure may lead to changes in the way the brain functions.

This is the first study to target key brain regions in the deep subcortical gray matter of chronic marijuana users with structural MRI and to correlate abnormalities in these regions with an impaired working memory. Working memory is the ability to remember and process information in the moment and - if needed - transfer it to long-term memory. Previous studies have evaluated the effects of marijuana on the cortex, and few have directly compared chronic marijuana use in otherwise healthy individuals and individuals with schizophrenia.

The younger the individuals were when they started chronically using marijuana, the more abnormally their brain regions were shaped, the study reports. The findings suggest that these regions related to memory may be more susceptible to the effects of the drug if abuse starts at an earlier age.

"The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it," said lead study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine. "With the movement to decriminalize marijuana, we need more research to understand its effect on the brain."

The paper will be published Dec. 16 in the journal Schizophrenia Bulletin.

In the U.S., marijuana is the most commonly used illicit drug and young adults have the highest - and growing - prevalence of use. Decriminalization of the drug may lead to greater use.

Because the study results examined one point in time, a longitudinal study is needed to definitively show if marijuana is responsible for the brain changes and memory impairment. It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse. But evidence that the younger a subject started using the drug the greater his brain abnormality indicates marijuana may be the cause, Smith said.