A federally funded study suggests that marijuana legalization in California led to a “substitution effect,” with young adults significantly reducing alcohol and cigarette use after cannabis reform. Published in the Journal of Psychoactive Drugs, the research contradicts prohibitionist arguments, revealing no significant increase in marijuana use among young adults below the legal age for dispensaries. The study involved surveys of people aged 18-20 in the City of Los Angeles before and after adult-use marijuana legalization in 2016. While there was a shift to edibles post-legalization, the overall frequency of cannabis use did not increase. Notably, there were fewer alcohol and cigarette use days in the post-legalization cohort, suggesting a possible protective effect or changes in norms. The study, funded by the National Institute on Drug Abuse, also found that changes in illicit and prescription drug use did not significantly differ between pre- and post-legalization cohorts, challenging the “gateway theory.” The findings align with other studies suggesting a substitution effect and reduced opioid use associated with marijuana legalization. The study recommends monitoring these trends as participants age.