OPRK1 and Respiratory insufficiency due to muscle weakness: Selective KOR agonists do not have the common side effects elicited by MOR agonists, such as tolerance, addiction, respiratory depression and constipation, but often trigger central nervous system (CNS)-mediated side effects such as hallucinations, sedation, diuresis and dysphoria (Viscusi et al., 2021; Shram et al., 2022).