U-50488 is a drug which acts as a highly selective κ-opioid agonist, but without any μ-opioid antagonist effects.[1] It has analgesic, diuretic and antitussive effects,[2] and reverses the memory impairment produced by anticholinergic drugs.[3] U-50488 was one of the first selective kappa agonists invented and research on its derivatives has led to the development of a large family of related compounds.[4][5]