As mental health gains prominence in public health discourse, scientific literature on the relationship between recreational cannabis use and mental health outcomes remains mixed. How does the legalization of recreational cannabis affect mental health-related hospitalizations, and how do these outcomes vary across diagnoses and subpopulations? While cannabis use has been linked to disorders such as psychosis and depression, it has also demonstrated therapeutic and stress-relieving effects. Despite ongoing debate, much of the existing scientific and economic literature is inconclusive, outdated, or lack- ing nuance. This study compiles results across seven mental health classification groups, finding that the legalization of recreational marijuana—particularly the start of sales in 2019—was associated with a highly statistically significant reduc- tion of approximately 75 psychosis-related hospitalizations per 100,000 people in Massachusetts relative to Rhode Island (p<0.01). Using a differences-in-differences regression, this study observes parallel pre-treatment trends that provide a strong preliminary case for further causal research on the mental health impact of cannabis legalization.