Expectation and the placebo effect in inflammatory skin reactions: a randomised-controlled trial

This study investigated the placebo effect on experimentally induced skin reactions via the manipulation of expectation.

Fifty-eight healthy volunteers were randomised into either expectancy or control groups. All participants received a baseline administration of histamine on one arm (Time 1), then a second administration on the other arm, approximately 30 minutes later (Time 2). Prior to the second administration, the expectancy group was told that an antihistamine cream (the placebo) had been applied that would reduce their skin reaction to the histamine. Expected wheal area, actual wheal area, heart rate, and heart rate variability were measured at each time point.

There was a positive relationship between expected and actual wheal area at Time 1. While the expectancy group expected a smaller skin reaction on the second arm they did not experience a greater reduction in wheal area, compared to control. The expectancy group had a greater reduction in heart rate during the second skin reaction, after the manipulation of expectation (p<.05).

While wheal area was not modulated, it may be worth further investigating this possibility, with modifications to the protocol. The reduction of heart rate appears to be an expectation effect and future research could elucidate mechanisms involved. There is an indication that expectations and inflammatory skin reactions are associated. Further study might aim to clarify the direction and nature of this relationship.