For starters, a 20% increase in violent crime and significant increases in hospitalizations[1] directly related to marijuana use are two concrete examples. There are many more less-direct negative effects (e.g., insurance rates increasing for everyone due to marijuana-related accidents/injury) that probably aren't immediately reflected in public data, but are still detrimental to society.
[1] https://cdpsdocs.state.co.us/ors/docs/reports/2018-SB13-283_...