The U.S. Supreme Court’s 2019 decision in Azar v. Allina Health Services 1 effectively curtailed the enforceability of certain Medicare policies established without notice-and-comment rulemaking. As a result, health care fraud cases brought under the False Claims Act (FCA), which are predicated on sub-regulatory guidance, face heightened scrutiny after Allina. 2 In a recent Advisory Opinion, the Health and Human Services Office of the General Counsel (HHS-OGC) provided its interpretation of Allina and how violations of sub-regulatory guidance affect the ability of governmental enforcement agencies and relators to bring and support FCA actions moving forward. 3 Specifically, on 3 December 2020, HHS-OGC issued Advisory Opinion 20-05 on Implementing