Difelikefalin (CR845, KorsuvaTM; Cara Therapeutics, Stamford, CT, USA) is a selective kappa opioid receptor (KOR) agonist designed to limit migration to the central nervous system to regulate pruritus, visceral and inflammatory pain, and inflammatory signals and to reduce the occurrence of insomnia by acting primarily on KORs in the peripheral and immune systems [11–13]. Here, OPRK1 is linked to insomnia.