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 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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