Ciprefadol is an opioid analgesic that is an isoquinoline derivative most closely related to cyclazocine and picenadol,Dennis M. Zimmerman et al. N-cycloalkylmethyl decahydroisoquinolines. US Patent 4001248, Jun 7, 1974 with a number of other related compounds known.David R. Brittelli et al. 4a-Aryl-trans-decahydroisoquinolines. US Patent 4419517, Apr 8, 1975Henry Rapoport et al. Synthesis of 4A-aryl-decahydroisoquinolines. US Patent 4189583, Apr 26, 1978 Ciprefadol is a mixed agonist–antagonist at μ-opioid receptors and can partly block the effects of morphine at low doses, though at higher doses it acts more like a full agonist. It is also a potent κ-opioid agonist, unlike the corresponding N-methyl and N-phenethyl derivatives which are reasonably μ-selective agonists.
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