Background: There is a striking geographic variation in drug overdose deaths without a specific drug recorded, many of which likely involve opioids. Knowledge of the reasons underlying this variation is limited.Objectives: We sought to understand the role of medicolegal death investigation (MDI) systems in unclassified drug overdose mortality.Methods: This is an observational study of 2014 and 2018 fatal drug overdoses and U.S. county-level MDI system type (coroner vs medical examiner). Mortality data are from the CDC's National Center for Health Statistics. We estimated multivariable logistic regressions to quantify associations between MDI system type and several outcome variables: whether the drug overdose was unclassified and whether involvement of any opioid, synthetic opioid, methadone, and heroin was recorded (vs unclassified), for 2014 (N = 46,996) and 2018 (N = 67,359).Results: In 2018, drug overdose deaths occurring in coroner counties were almost four times more likely to be unclassified (OR 3.87, 95% CI 2.32, 6.46) compared to medical examiner counties. These odds ratios are twice as large as in 2014 (difference statistically significant, P < .001), indicating that medical examiner counties are improving identification of opioids in drug overdoses faster than coroner counties.Conclusions: Accurate reporting of drug overdose deaths depends on MDI systems. When developing state policies and local interventions aimed to decrease opioid overdose mortality, decision-makers should understand the role their MDI system is playing in underestimating the extent of the opioid overdose crisis. Improvements to state and county MDI systems are desirable if accurate reporting and appropriate policy response are to be achieved.