Pain experience, possession of analgesic boost pain tolerance, threshold

Prior exposure to pain and the mere possession of an analgesic seem to affect subsequent analgesia, altering pain threshold and tolerance, a new study has found.

The researchers randomly allocated 127 healthy participants to a prior pain (PP) or no-PP condition. Those in the PP group underwent a preliminary cold pressor test (CPT) trial, after which all participants in both groups were again randomized to receive either an analgesic cream or a pain-irrelevant anti-itch cream as a control. Participants then underwent the main CPT, from which the researchers measured several pain outcome parameters.

According to a 2 × 2 multivariate analysis of variance, prior pain experience had a significant main effect on real-time pain outcomes (pain threshold, pain tolerance, and mean numerical rating scale [NRS]; p<0.001). Analgesia possession status, on the other hand, had no significant main effect (p=0.21). Notably, the interaction effect of the two factors was significant (p<0.001).

Bonferroni posthoc analysis showed that the possession of an analgesic had a significant effect on pain threshold in no-PP participants, but not in the PP group. Among participants with no prior experience of a pain challenge, those given the analgesic cream had higher pain threshold than those who had been randomized to the control analgesic. A similar effect was reported for pain tolerance.

However, such mere-possession effects were absent in the PP group.

“[O]ur study showed that the observed placebo effect is experience-dependent,” the researchers said. “[F]uture studies should be aware that having prior pain experience with cold water would hinder placebo analgesia using CPT regardless of whether the prior pain was naturally occurred or experimentally induced.”