What Is the Nocebo Effect?

From Smithsonian:

What if taking an absolutely harmless substance could make you sick? What if a sugar pill caused you to feel nausea, or a fake dose of lactose triggered unwelcome stomach symptoms in patients who are lactose intolerant? The strange truth about medicine and the brain is that they often interact in completely unpredictable and counterintuitive ways. Nowhere is this more true than with the bewildering phenomenon known as the nocebo effect. Most of us already know about the placebo effect. As part of medical studies, a control group is typically given an inert substance (usually a sugar pill) that provides a baseline to which researchers can compare the effectiveness of the new medicine being tested. The members of this group are told that the placebo is real—and surprisingly, they sometimes experience an actual improvement in their symptoms, simply because they expect that the medicine will make them feel better.

An opposite tendency—and one that has been largely overlooked by the research community—is the nocebo effect. Put simply, it is the phenomenon in which inert substances or mere suggestions of substances actually bring about negative effects in a patient or research participant. For some, being informed of a pill or procedure’s potential side effects is enough to bring on real-life symptoms. Like the placebo effect, it is still poorly understood and thought to be brought about by a combination of Pavlovian conditioning and a reaction to expectations. Last week, researchers from the Technical University of Munich in Germany published one of the most thorough reviews to date on the nocebo effect. Breaking down 31 empirical studies that involved the phenomenon, they examined the underlying biological mechanisms and the problems it causes for doctors and researchers in clinical practice. Their conclusion: although perplexing, the nocebo effect is surprisingly common and ought to be taken into consideration by medical professionals on an everyday basis.

More here.