Comparing the subjective, psychomotor and physiological effects of intravenous nalbuphine and morphine in healthy volunteers

The purposes of this study were to characterize the subjective, psychomotor and physiological effects of nalbuphine in healthy non-drug abusing volunteers and to compare and contrast the effects of equianalgesic doses of nalbuphine and morphine. Subjects (12 males, 4 females) without histories of opiate dependence were injected in an upper extremity vein with 0, 2.5, 5.0 or 10 mg/70 kg nalbuphine, or with 10 mg/70 kg morphine, using a randomized, double-blind, crossover design. The 10-mg doses of nalbuphine and morphine are considered equianalgesic and are doses commonly given for relief of postoperative pain. Subjective effects of nalbuphine included increased scores on the Pentobarbital-Chlorpromazine-Alcohol Group scale and the Lysergic Acid Diethylamide scale of the Addiction Research Center Inventory; increased adjective checklist ratings of "nodding," "numb" and "sweating"; increased visual analog scale ratings of "coasting or spaced out," "high" and "sleepy" and increased "feel drug effect" and drug-liking ratings. Ten milligrams of nalbuphine had subjective effects similar, and similar in magnitude, to those of 10 mg of morphine. Nalbuphine produced exophoria and impairment on the Digit Symbol Substitution Test in a dose-related fashion. Ten milligrams of morphine produced exophoria but did not affect performance on the Digit Symbol Substitution Test. Both nalbuphine and morphine induced miosis and decreases in respiration rate. The results of the present study demonstrate that 2.5 to 10 mg nalbuphine had orderly, dose-related effects on subjective, psychomotor and physiological variables. The results also indicate that 10 mg of nalbuphine produces a profile of subjective, psychomotor and physiological effects similar to that of an equianalgesic dose of morphine (10 mg). The similarity in profiles between drugs at this dose is consistent with both infrahuman studies, which suggests that nalbuphine is a mu agonist, and studies with nondependent opioid abusers, in which relatively low doses of nalbuphine (such as 10 mg) produce morphine-like effects.