Opioid overdose is the most common cause of accidental death in the United States and no policy response has been able to contain this epidemic to date. We examine whether local access to medical cannabis can reduce opioid-related mortality. Using a unique data set of medical cannabis dispensaries combined with county-level mortality data, we estimate the effect of dispensaries operating in a county on the number of overdose deaths. We find that counties with dispensaries experience 6% to 8% fewer opioid-related deaths among non-Hispanic white men. Mortality involving heroin declines by approximately 10% following the opening of a dispensary.