Ciprefadol is an opioid analgesic that is an isoquinoline derivative most closely related to cyclazocine and picenadol, with a number of other related compounds known. Ciprefadol is a mixed agonistâÂÂantagonist at ü-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 ú-opioid agonist, unlike the corresponding N-methyl and N-phenethyl derivatives which are reasonably ü-selective agonists.