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OBJECTIVE In this study we assess whether changes in ZIP code-level density of medical marijuana facilities are related to changes in rates of opioid poisonings and opioid use disorder hospitalizations in California. METHOD A panel study using California hospital discharge data was conducted to assess the relationship between density of medical marijuana dispensaries and opioid poisonings and use disorder. There were 8,536 space-time units at the ZIP code level. Outcome measures included ZIP code counts of opioid poisonings and opioid use disorder; independent variables were local- and adjacent-area medical marijuana dispensaries and demographic and economic characteristics. RESULTS Independent of effects for covariates, densities of medical marijuana dispensaries were positively related to opioid use disorder (RR = 1.05, CI [1.03, 1.06]) and opioid poisonings (RR = 1.04, CI [1.02, 1.05]) in local areas, but negatively related to opioid misuse in spatially adjacent areas (RR = 0.91, CI [0.88, 0.94] for opioid use disorder, RR = 0.89, CI [0.86, 0.93] for opioid poisonings). CONCLUSIONS Although state-level studies suggest that more liberal marijuana policies may result in fewer opioid overdose deaths, our results within one state suggest that local availability of medical marijuana may not reduce those deaths. The relationship appears to be more complex, possibly based on socioeconomic conditions within and adjacent to areas with higher densities of medical marijuana dispensaries.