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With the use of legal marijuana proliferating, many want to understand the potential risks to teen users in particular. But thus far, definitive answers about the drug’s effect on young brains have been hard to come by.

For a developmental neuroscientist, Kuei Y. Tseng gets recruited to deliver a lot of talks to the public. Parents, educators, law enforcement, and teenagers all want Tseng to educate them about a hot-button issue: how cannabis affects the adolescent brain.

Tseng, based at the University of Illinois in Chicago, investigates how rats respond to THC (tetrahydrocannabinol), the main psychoactive ingredient in cannabis. He’s found that exposure to THC or similar molecules during a specific window of adolescence delays maturation of the prefrontal cortex (PFC), a region involved in complex behaviors and decision making (1). The disruption alters how the area processes information when the animals are adults.

Audiences of parents and teachers tend to be alarmed by the long-lasting deficits, Tseng says. But when he speaks with teenagers, especially those already using cannabis, he gets a different response. “It’s surprising, but they’re not that worried.” Youths often want to know how much cannabis they can consume without harming brain development; they press Tseng to extrapolate from rats the age at which it’s “safe” for people to start using the drug. “They need to understand this is not black and white,” says Tseng.