The percentage of hospitalized patients on chronic medication for opioid use disorder (MOUD) is increasing, and hospital providers must therefore understand the fundamental concepts relating to managing pain in this high-risk population [57] Published evidence and experience support the continuation of methadone (mu-opioid receptor agonist and NMDA antagonist) and buprenorphine (partial mu-opioid receptor agonist and kappa-opioid receptor antagonist) regimens during acute painful episodes, including through surgical encounters. The gene discussed is OPRM1; the disease is opioid use disorder.