Teens who are heavy marijuana users show abnormal changes in their brain structures that are associated with having schizophrenia, a new study has found.
Researchers from the Northwestern University found that chronic smokers, those who smoked marijuana daily for about three years, showed changes in their brain structures related to working memory and performed poorly on memory tasks.
Researchers observed the brain abnormalities and memory problems 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 showed the marijuana-related brain abnormalities are correlated with a poor working memory performance and look similar to schizophrenia-related brain abnormalities.
Of the 15 marijuana smokers who had schizophrenia in the study, 90 per cent started heavily using the drug before they developed the mental disorder, researchers said.
Marijuana abuse has been linked to developing schizophrenia in prior research.
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.
The younger the individuals were when they started chronically using marijuana, the more abnormally their brain regions were shaped, the study found.
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 behavioural sciences at Northwestern University Feinberg School of Medicine.
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, Smith said.
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.
The study was published in the journal Schizophrenia Bulletin.