The nocebo effect

The nocebo effect is how the expectation of pain or negative effects of a treatment increases the chances people will feel them — even if they're aren't any. So, tell someone that a treatment will hurt or have side-effects and they're now more likely to feel that pain or find something wrong.

Taken together with the placebo effect I guess we might do well, for normal affairs, to let people know that what they will get will probably help them and not raise unnecessary expectations that something might hurt.