A sugar pill can change the body

Placebo analgesia is partly opioid. A drug that blocks opioid receptors also blocks some of the pain relief of a sham treatment.

Levine, Gordon, and Fields gave dental patients a placebo and told them it was a painkiller. It worked, on average. Then they gave naloxone, which blocks opioid receptors. A substantial part of the placebo relief went away. The expectation had recruited the brain’s own opioids.

Placebo is a response to context: words, rituals, conditioning. It does not shrink a tumour because you wished. It does turn down pain through named pathways.