OPRM1 and respiratory depression: Likewise, nalbuphine, a moderate efficacy MOR partial agonist and high efficacy KOR partial agonist (Schmidt et al., 1985; Peng et al., 2007), is similar in analgesic efficacy to morphine, but shows negligible respiratory effects (Schmidt et al., 1985), suggesting that partial agonism coupled to mixed agonism at MOR/KOR may attenuate MOR-induced respiratory depression.