How side effects can improve treatment efficacy: a randomized trial

Volunteers were told they might get a strong painkiller nasal spray before some heat-pain tests. In fact, none of the sprays contained painkiller: some had a pinch of chilli extract to cause a mild burning feeling, and others were just salt water.

The sprays with a side effect led to less pain than the plain ones. A little tingle seems to whisper “this is definitely working”, and the brain’s own pain-dampening system listens.