Pill mill laws impose strict regulations on pain management clinics to prevent them from issuing opioid prescriptions without medical indication. To date, little is known about the implementation or effects of these laws on opioid overdose deaths. A previously untested concern is that by restricting access to prescription opioids, pill mill laws could increase overdose from heroin and synthetic opioids, like illicitly produced fentanyl. We evaluated the effects of pill mill laws on opioid overdose deaths in Ohio and Tennessee. Of the 11 total U.S. states with pill mill laws, Ohio and Tennessee were the only two where: (1) the pill mill law was the only state law designed to curb opioid prescribing implemented in a two-year period, one-year pre/post law; and (2) high-quality drug-specific overdose death data were available from CDC. We conducted synthetic control analyses examining differences in post-pill mill law trends in overdose deaths in Ohio and Tennessee compared to weighted combinations of comparison states. We also conducted qualitative interviews with 11 leaders responsible for pill mill law implementation and enforcement in Ohio and Tennessee. Pill mill law enactment had no effects on overall, prescription opioid, heroin, or synthetic opioid overdose deaths in Ohio or Tennessee. Interview results suggest that both states engaged in robust enforcement and implementation of the law. A multi-pronged policy approach, including but not limited to pill mill laws, may be required to effectively address opioid overdose deaths.