Chronic teen cannabis use may negatively impact brain's reward system, increase risk of addiction

PROVIDENCE, R.I. [Brown University] - A new study shows that among teens who use cannabis, the drug may interfere with the brain's reward system at a time of crucial development, and higher-potency cannabis products have more pronounced effects.

"Adolescence is a critical window for brain development," said lead study author Sarah A. Thomas, an assistant professor of psychiatry and human behavior (research) at Brown University's Warren Alpert Medical School and a clinical psychologist at Brown University Health's Bradley Hasbro Children's Research Center. "Our findings suggest that repeated cannabis use during this period has the potential to alter the dopamine system in ways that could affect motivation, reward processing and vulnerability to addiction."

The study was published in Neuropsychopharmacology.

Previous research has found that approximately 10% to 20% of U.S. adolescents report using cannabis, and studies in humans and animals provide strong evidence that adolescence is a vulnerable period to start using the drug because it interferes with the brain's development process. Cannabis use during the teenage years, compared to adulthood, is linked to a higher risk of developing cannabis use disorder and experimenting with other drugs. One possible reason is that cannabis disrupts the brain's dopamine system - the circuits that help regulate motivation, learning and reward.

In a federally funded study with 81 participants aged 14 to 17, the researchers assessed cannabis use quantity, frequency and cannabis use disorder severity. They then used magnetic resonance imaging to measure tissue iron in brain regions with high dopamine activity. Tissue iron is a necessary factor in the production of dopamine and naturally increases during adolescence as the dopamine system matures, the researchers said, making it a useful indicator of healthy development.

The study found that in many brain regions, adolescents who use cannabis had significantly lower tissue iron levels than those with little to no use. Results showed that cannabis use quantity, frequency, hours spent under the influence and cannabis use disorder severity were negatively associated with markers for tissue iron.

The researchers say the study is the first to examine how cannabis use in teens relates to tissue iron levels in the brain - a reliable, noninvasive marker linked to dopamine activity. The results, they note, add to growing evidence that cannabis use by teens may have negative effects on brain development, making individuals who use cannabis more vulnerable to addiction.