Here's a summary:

the "nocebo effect" - the flip-side to the better-known placebo effect. While an inert sugar pill (placebo) can make you feel better, warnings of fictional side-effects (nocebo) can make you feel those too... This poses an ethical quandary: should doctors warn patients about side-effects if doing so makes them more likely to arise?

Examples given include- A man died shortly after being told he had cancer, even though autopsy showed the cancer hadn't grown, and didn't show any other reason for his death. Another example is mass psychogenic illnesses such as when 62 factory workers got very sick due to an "insect" that was never found. This spreads most rapidly to female individuals who have seen someone else suffering from the condition. Finally, telling someone that a pain-killing drug has worn off (even if it hasn't yet) is enough to return them to pre-drug levels of pain.

when volunteers feel nocebo pain, corresponding brain activity is detectable in an MRI scanner. This shows that, at the neurological level at least, these volunteers really are responding to actual, non-imaginary, pain

one of the neurochemicals responsible for converting the expectation of pain into this genuine pain perception... is called cholecystokinin and carries messages between nerve cells. When drugs are used to block cholecystokinin from functioning, patients feel no nocebo pain, despite being just as anxious...Benedetti's work on blocking cholecystokinin could pave the way for techniques that remove nocebo outcomes from medical procedures, as well as hinting at more general treatments for both pain and anxiety.

Doctors often stress possible side-effects to avoid getting sued, but lowering a patient's confidence in a treatment is counter-productive.