The kappa opioid nalbuphine produces gender- and dose-dependent analgesia and antianalgesia in patients with postoperative pain.

Nalbuphine, pentazocine, and butorphanol, mixed agonist/antagonist opioids that induce analgesia by acting predominantly at kappa opioid receptors, have recently been shown in single-dose studies to have greater analgesic efficacy in women than in men. In the current experiments, the first placebo controlled dose response study of opioid analgesic efficacy that examines for gender differences, nalbuphine (5, 10, or 20 mg) and placebo were evaluated in 62 men and 69 women for the treatment of moderate to severe postoperative pain following extraction of impacted wisdom teeth. In a randomized, open injection, double blind experimental design, pain intensity was recorded on a 10 cm visual analog scale (VAS) immediately prior to drug administration (baseline) and at 20 min intervals thereafter. Although responses to placebo were similar in men and women, for all doses of nalbuphine women exhibited significantly greater analgesic response than men, compatible with our previous results. Unexpectedly, men receiving the 5 mg dose of nalbuphine experienced significantly greater pain than those receiving placebo; only the 20 mg dose of nalbuphine in men produced significant analgesia compared to placebo. While a similar antianalgesic effect was not observed in women, only the 10 mg dose of nalbuphine produced significant analgesia compared to placebo. These results suggest that the optimal analgesic dose of nalbuphine for women is lower than the highest dose that can be safely administered. In contrast, the antianalgesic effect of nalbuphine suggests avoidance of its routine use for postoperative analgesia in men until further studies clarify this issue. Because gender differences in other mixed kappa agonists/antagonists (i.e. pentazocine and butorphanol) have previously been shown, these results may generally apply to this class of opioid analgesics.