Summary: The evil twin of the placebo effect is the nocebo effect, when you give an inert substance and get an adverse reaction. Note that is different than a side effect. I give a patient penicillin and they get a rash, that is a side effect. I give them a product with no effect and they get a complication, that is a nocebo effect. "A nocebo (Latin for “I will harm”) is something that should be ineffective but which causes symptoms of ill health. A nocebo effect is an ill effect caused by the suggestion or belief that something is harmful." It is why I was amused by Clinically meaningful nocebo effect occurs in acupuncture treatment: a systematic review.
