What counts as heavy pot use

Heavy use of pots can damage the brain

Prolonged daily use of marijuana appears to adversely affect brain structures and cause memory impairment.

The researchers found that adolescents who used marijuana heavily and smoked daily for about three years had abnormal changes in their brain structures related to working memory and performed poorly on memory tasks.

The Northwestern Medicine study found that the brain abnormalities and memory problems persisted two years after quitting marijuana smoking.

Poor working memory predicts poor academic performance and everyday functions.

The brain abnormalities and memory problems were observed in the early 20s, which could indicate the long-term effects of chronic use.

Memory-related structures in her brain seemed to shrink inward and collapse, possibly due to a decrease in neurons.

The study also shows that the marijuana-related brain abnormalities correlate with poor working memory performance and are similar to the brain abnormalities related to schizophrenia.

Over the past decade, Northwestern scientists, along with scientists from other institutions, have shown that changes in brain structure can lead to changes in brain function.

The researchers say this is the first study to use structural MRI to examine key brain regions in the deep subcortical gray matter of chronic marijuana users, and to correlate abnormalities in those regions with impaired working memory.

Working memory is the ability to store and process information in the moment and, if necessary, to transfer it to long-term memory.

Previous studies have looked at the effects of marijuana on the cerebral cortex, and few have directly compared chronic marijuana use in otherwise healthy individuals and those with schizophrenia.

The younger the individuals were when they started chronic marijuana use, the more unusual their brain areas became, the study reports.

The results suggest that these memory-related regions may be more prone to the drug's effects if the abuse starts at an earlier age.

"The study has linked chronic marijuana use to brain disorders that appear to persist for several years after people stop using marijuana," said the study's senior author Matthew Smith.

"With the movement to decriminalize marijuana, we need more research to understand its effects on the brain."

The research results are published in the journal Schizophrenia Bulletin.

In the United States, marijuana is the most widely consumed illicit drug, and young adults have the highest - and growing - prevalence of use. Decriminalizing the drug can lead to increased consumption.

Since the study results were examined at a specific point in time, a longitudinal study is needed to definitively show whether marijuana is responsible for the brain changes and impaired memory.

It is possible that the abnormal brain structures indicate a pre-existing susceptibility to marijuana abuse. But the younger a subject started taking the drug, the greater the abnormality in their brain, suggesting that marijuana could be the cause, Smith said.

The groups in the study started using marijuana every day between the ages of 16 and 17 for about three years. At the time of the study, they had been without marijuana for about two years.

A total of 97 subjects participated, including matched groups of healthy controls, subjects with a marijuana use disorder, schizophrenia subjects with no history of substance use disorder, and schizophrenia subjects with a marijuana use disorder.

The subjects who used marijuana did not abuse any other drugs.

Few studies have looked at the effects of marijuana on the deep regions of the brain, the “subcortical gray matter” beneath the noodle-shaped cortex.

The study is also unique in that it examines the shapes of the striatum, globus pallidus and thalamus, structures in the subcortex that are critical to motivation and working memory.

Chronic marijuana use can contribute to brain structure changes associated with schizophrenia, Northwestern research shows.

Of the 15 marijuana smokers who suffered from schizophrenia in the study, 90 percent began using the drug intensely before developing the mental disorder. Marijuana abuse has been linked to the development of schizophrenia in previous research.

"Abusing popular street drugs like marijuana can have dangerous consequences for young people who develop or have developed mental disorders," said co-senior study author John Csernansky, MD

"This paper is one of the first to show that marijuana use can contribute to brain structure changes that have been linked to schizophrenia."

Chronic marijuana use could aggravate the underlying disease process associated with schizophrenia, Smith noted.

"If someone in the family has schizophrenia, their risk of developing schizophrenia increases if they abuse marijuana," he said.

While both chronic marijuana smokers and chronic marijuana smokers with schizophrenia had brain changes related to the drug, patients with mental disorders had greater thalamus deterioration.

This structure is the brain's communication center and is crucial for learning, memory and communication between brain regions. The brain regions examined in this study also have an effect on motivation, which is already noticeably impaired in people with schizophrenia.

"A tremendous amount of addiction research has focused on regions of the brain traditionally associated with reward / aversion function and thus motivation," noted co-senior study author Hans Breiter, MD