Forty states mandate that providers query their patients' prescription histories in the state's prescription drug monitoring program (PDMP) before prescribing controlled substances. However, little is known about providers' use of PDMPs, either with or without a mandate. We measured the share of opioid prescriptions with PDMP queries in Kentucky from 2010 to 2018, before and after the implementation of the first comprehensive PDMP mandate in the US. Providers queried the PDMP for 12 percent of opioid prescriptions before the mandate; after the mandate, they queried for 56 percent of prescriptions. The share of prescriptions queried was lowest for patients without recent opioid use (3 percent before the mandate, 25 percent after) and highest for pain management specialists (31 percent before, 72 percent after). Over time, high-compliance providers reduced prescribing to the riskiest patients, whereas low-compliance providers continued to prescribe to them. Although the share of prescriptions queried greatly increased after the mandate, compliance remained incomplete, including for patients with high-risk patterns of opioid use.