Can marijuana affect teen brain development? What we know some negative effects of marijuana on teen brain development have been shown. The prefrontal cortex does not reach full maturity until the early twenties, meaning that many complex processes of the adolescent brain are still in progress. The substance abuse and mental health agency reports that marijuana is the most commonly used illicit substance in the united states, according to results from the 2018 national survey on drug use and health. 5 million individuals reported using marijuana during the past year. Unlike adults, the teen brain is actively developing and often will not be fully developed until the mid 20s. Marijuana use during this period may harm the developing teen brain. Negative effects include difficulty thinking and problem solving. marijuana may hurt the developing teen brain shots - health news studies show regular marijuana use by teens once a week or more affects. an important topic with marijuana is a potential risk to brain development.