@Sir Runcible Spoon
Nocebo effect? I think we may have been reading the same books :-)
They're not sure but it's a fascinating topic, precursor chemicals detected in the brain have been isolated and attributed to the placebo and the nocebo effect, it isn't all fluff and bunnies, there's some real science in there.
Half the anti-depressants on the market would NOT pass placebo testing today.
You can take patients on courses high morphine and slowly lower them to a saline solution and if the patient is told he is still on morphine they will feel no pain, this seems to work on about 2/3 of people. Then, without telling them, you can push a dose of Naloxone (an opiate blocker) into that saline solution and they WILL FEEL PAIN once again.
The jury is out on the placebo/nocebo effect, but it's fascinating stuff.