Ciprefadol is an opioid analgesic drug. It binds with a high affinity to mu (Ki 4.2 nM) and kappa (Ki 2.5 nM) opioid receptors. In vivo, ciprefadol displays mixed antagonist/agonist activity in the mouse writhing and the rat tail heat tests: in low doses, the compound inhibits the analgesic effect of morphine, while at higher doses it displays analgesic effect. Chronic administration of ciprefadol to rhesus monkeys produced a marked physical dependence more severe than that of morphine, and its effect was consistent with what would be expected of a potent, long-lasting morphine-like agonist.