As recreational cannabis legalization spreads across U.S. states, concerns about health consequences—particularly for vulnerable populations and developing adolescent brains—are mounting, according to Deepak D’Souza, the Vikram Sodhi Professor of Psychiatry at Yale School of Medicine and director of the Yale Center for the Science of Cannabis and Cannabinoids.
Today’s cannabis is vastly more potent than previous decades. Back in the 1960s, THC content was about 3 to 4%. In 2018 it was closer to 18%. But dispensaries now sell cannabis with 35% THC, and concentrates can reach up to 95% THC—20 times more potent than what previous generations smoked. Connecticut caps herbal material at about 35% THC with concentrates at 70%, but each state has its own rules, creating inconsistent regulation.
D’Souza has long observed negative impacts on patients with serious mental illness: people with schizophrenia who use cannabis are more likely to end up in emergency rooms or hospitalized. Predicting who will develop such illnesses is difficult; while family history is an obvious risk factor, many young people destined to develop serious mental illness may have no known family history. Recent reports have linked cannabis use to elevated risks of heart disease and stroke, brain developmental disorders, and spikes in emergency visits. In Massachusetts, which legalized recreational marijuana in 2016, a statewide referendum campaign to reverse legalization has gained support.
The adolescent brain is particularly vulnerable. Mid-adolescence poses much greater risks than cannabis use in young adulthood. The endocannabinoid system regulates learning, memory, emotional processing, sleep, and neurodevelopment, which occurs primarily in the fetal brain and during adolescence. When the developing brain is “bombarded by THC during adolescence,” it can have “far reaching implications.” A staggering array of products now appeals to young people: gummies, candies, and cookies loaded with THC. D’Souza emphasizes that legalization isn’t so much about legalization as commercialization, driven by commercial enterprises and governments earning revenue. Without education, surveillance programs monitoring psychiatric outcomes, and regulation limiting THC potency and youth access, cannabis harms will continue mounting.
This article is an AI-assisted summary. All facts and figures are drawn from the original report: https://news.yale.edu/2026/02/06/legal-weed-lingering-concerns