Most people are familiar with placebos. These are inert substance that create a positive response (or no response). The phenomenon in which a placebo creates a positive response is called the placebo effect. Most people haven't heard of nocebos. These are harmless substances that create harmful effects in patients who take them. In short, the nocebo effect is the negative reaction experienced by a patient who receives a nocebo.
Rather than being caused by a biologically active compound in the nocebo or placebo, these reactions result from a subject's expectations about how the substance will affect him or her. Though they originate exclusively from psychological sources, nocebo effects can be either psychological or physiological.
Learn more about this amazing phenomenon in the video below.