OPRM1 and Respiratory insufficiency due to muscle weakness: Blockade of endogenous opioid peptide catabolism in vivo does not cause respiratory depression, raising the possibility that a positive allosteric modulator at MOR will generate analgesia without causing respiratory depression (Boudinot et al., 2001; Fournie-Zaluski et al., 1984; Kayser et al., 1989).