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). This evidence concerns the gene OPRK1 and respiratory depression.