OPRM1 and respiratory depression: U-50488 [trans-3,4-dichloro-N-methyl-N-[ 2-(1-pyrrolidinyl) cyclohexyl]-benzeneacetamide] is a kappa-opioid receptor agonist, with some reported mu-opioid receptor respiratory antagonist effects.124,133 Studies in animals have shown that U-50488 causes diuresis and dysphoria rather than respiratory depression or constipation.134 The toxicological profile of U-50488 currently remains under research, but the structural similarity to U-47700 suggests that it might pose a significant risk.135