Doctors Xiulu Ruan and Shakeel Kahn were convicted of unlawfully dispensing controlled substances based on prescriptions the Government alleged were issued outside legitimate medical practice. The Supreme Court held that the controlled-substances statute requires the Government to prove that a physician knowingly or intentionally acted in an unauthorized manner. A doctor’s good-faith belief that a prescription was issued for a legitimate medical purpose can defeat the required mens rea, even if the prescription objectively departed from medical standards. The Court rejected an objective-only standard that would permit conviction based solely on what a reasonable physician should have known. It vacated the judgments of the Courts of Appeals and remanded for those courts to determine whether the trial instructions correctly described the good-faith defense and whether any instructional errors were harmless.