Past pain may interfere with placebo analgesia

The mere possession of a placebo analgesia shows a clear pain suppression effect, but only in those without a recent pain experience, a new trial has found.

The researchers randomly assigned 127 healthy individuals to possess either an inert cream described as an analgesic or an anti-itch cream. Participants were then asked to complete a cold pressor test (CPT). Prior to this, a random selection of participants had undergone a CPT, simulating a prior pain (PP) experience; the remaining participants were designated as the No-PP group.

Pain was measured through real-time measurements (pain threshold, pain tolerance, and numerical rating scale [NRS] scores) and retrospective self-reports, through the McGill Pain Questionnaire (MPQ).

A 2-by-2 multivariate analysis of variance (MANOVA) showed that prior pain experience had a significant main effect (p<0.001) on real-time pain measures, while the possession of a placebo analgesic was not (p=0.21). However, the interaction between prior pain experience and analgesic possession was significantly linked to pain (p<0.001).

In particular, the No-PP group performed significantly better in terms of pain threshold (p=0.05) and tolerance (p=0.002) than their PP counterparts. NRS scores were also much better in No-PP participants (p=0.002). Possession of a placebo analgesia likewise improved pain threshold (p=0.001) in those who did not experience prior pain.

In terms of retrospective pain outcomes, MANOVA showed that only prior pain (p=0.004) had a significant effect. The No-PP participants reported significantly lower MPQ pain severity (p=0.006), but there was no difference in terms of pain intensity and quality. The interaction between prior pain and analgesic possession had no impact on MPQ.

“The present study identifies a concise contextual framework that has implication for pre- and post-test experimental design in CPT,” the researchers said. “[O]ur study showed that the observed placebo effect is experience-dependent; future studies should be aware that having prior pain experience with cold water would hinder placebo analgesia.”