Gender and the placebo analgesic effect in acute pain

Our objective was to examine the placebo arms from a series of clinical trials in which the post-third molar extraction dental pain model was used to elucidate the time course of the placebo effect and the proportion of the population that are responders, as well as to evaluate whether the placebo analgesic response of female subjects may differ from that of male subjects.

We performed a meta-analysis of 596 subjects included in the placebo treatment arm of 16 double-blind, post-third molar extraction dental pain (moderate to severe) studies submitted to the Food and Drug Administration electronically. The inclusion and exclusion criteria were practically identical in all studies. Pain relief and pain intensity measurements used the same metrics in all studies. The measurements were recorded just before drug administration and at least at postdose hours 0.5, 1, 1.5, 2, 3, 4, 5, and 6.

There were 325 female subjects and 271 male subjects. They were all otherwise healthy, with a mean age of 21.6 years for female subjects and 22.3 years for male subjects. The postoperative baseline pain was greater in female subjects than in male subjects, and this difference was statistically significant. Both pain intensity and pain relief scores demonstrate the well-established placebo effect in 10% of the pooled subjects, as well as in all the individual studies. Over time, however, the mean pain intensity and pain relief scores for the female and male treatment groups were not noticeably different at any time point after medication. Further analysis of the data showed no gender difference in duration of action of the placebo.

The results demonstrated no gender difference in response to placebo. These results were obtained from the post-third molar extraction situation, in which the least possible confounding factors were present. To fully establish the generality of this phenomenon, studies should be carried out in other pain models.