Abstract

The subjective, physiological and behavioral effects of morphine, pentazocine and ciramadol, an opioid agonist/antagonist, were studied in adult male nondependent opioid abusers living on a clinical research ward. Fifteen subjects were assigned randomly to one of three groups. Each group received, by i.m. injection, placebo and three doses of one active drug, twice in randomized block order under double-blind conditions in 4.5-hr experimental sessions. Physiological measures did not differentiate between the three drugs. All three drugs decreased respiratory rate and pupil diameter and increased blood pressure. However, morphine, ciramadol and pentazocine produced different profiles on the subjective effect measures. All three drugs increased "liking," "good effects," "any effects" and "high" subjective effect scales. Pentazocine increased subjective "bad effects" scale scores and scales measuring dysphoria and sedation. Observers reported significant behavioral changes after administration of morphine and pentazocine, but not after ciramadol. Overall, the effects of morphine (7.5, 15 and 30 mg) and pentazocine (22.5, 45 and 90 mg) were dose-related. Although pentazocine produced increases in scales that indicated negative subjective effects, it also produced significant changes in most self-report measures that were increased by morphine, including liking and good effects scales. The effects of ciramadol were not dose-related, with all three doses (30, 60 and 120 mg) producing effects approximately equivalent to morphine 15 mg. Thus, ciramadol exhibited a ceiling effect typical of the opioid agonist/antagonist.