Heavy pot users -- smoking marijuana daily for three years -- had abnormal changes in their brain structures related to working memory, U.S. researchers say.
Lead study author Matthew Smith, an assistant research professor at Northwestern University Feinberg School of Medicine in Chicago, said poor working memory predicts poor academic performance and everyday functioning.
The groups in the study started using marijuana daily at ages 16 to 17 for about three years. At the time of the study, they had been marijuana free for about two years.
Almost 100 subjects participated, including matched groups of healthy controls, subjects with a marijuana use disorder, schizophrenia subjects with no history of substance use disorders, and schizophrenia subjects with a marijuana use disorder. The subjects who used marijuana did not abuse any other drugs, the researchers said.
Of the 15 marijuana smokers who had schizophrenia in the study, 90 percent started heavily using marijuana before they developed the mental disorder. Marijuana abuse has been linked to developing schizophrenia in prior research, Smith said.
"The abuse of popular street drugs, such as marijuana, might have dangerous implications for young people who are developing or have developed mental disorders," said co-senior study author Dr. John Csernansky of Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital.
"This paper is among the first to reveal that the use of marijuana may contribute to the changes in brain structure that have been associated with having schizophrenia."
Chronic marijuana use could augment the underlying disease process associated with schizophrenia, Smith noted.
"If someone has a family history of schizophrenia, they are increasing their risk of developing schizophrenia if they abuse marijuana," he said.
