Weed is harming teens brain development, largest US study warns

A new study has found sobering effects of weed on teenage brains.While more teens are staying away from marijuana across the country, the drug is still used by one in five kids in New York, according to a New York Impact Report from November 2025.And though some purport the substance can improve mood and energy, there are negative effects on cognitive skills — especially in younger users.Researchers from the University of California San Diego (UC San Diego) have conducted the largest long-term study in the US on cannabis use in kids.Analyzing more than 11,000 children from ages 9 to 17, the team tracked both substance use and cognitive performance through self-reporting and biological testing.Across a range of skills including memory, attention, language and processing speed, teens who used cannabis showed restricted growth during key years of brain development.Progress in memory, focus and thinking speed leveled off as they got older and started using cannabis compared to their peers who didn’t.And in smaller groups of participants, those exposed to THC had worsened memory over time, while CBD didn’t have the same effect.Although the differences were modest, researchers say they’re still important as they occurred during a crucial time of growth.“These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning,” lead study author and assistant professor in the Department of Psychiatry at UC San Diego School of Medicine Natasha Wade said in a press release.“[The result] also highlights how complicated cannabis products can be, especially since some products labeled as CBD may still contain THC,” Wade added.However, the study doesn’t prove a direct correlation between weed and cognitive impairment, as other factors can play a role, such as environment or personality.These findings come after marijuana poisoning cases have skyrocketed 56% among New Yorkers under 19, according to data from th...