As more states enact laws liberalizing marijuana use and the U.S. opioid epidemic surges to unprecedented levels, understanding the relationship between marijuana and opioids is growing increasingly important. Using a unique self‐constructed marijuana dispensary dataset, I estimate the impact of increased marijuana access on opioid‐related harms. I exploit within‐ and across‐state variation in dispensary openings and find county‐level prescription opioid‐related fatalities decline by 11% following the opening a dispensary. The estimated dispensary effects are qualitatively similar for opioid‐related admissions to treatment facilities. These results are strongest for males and suggest a substitutability between marijuana and opioids.