A few years ago, scores of middle-aged men took part in a clinical trial of a new drug, intended to ease the symptoms of their enlarged prostates. But this wasn't just a study of a drug's power to help. It was also a study of the mind's power to hurt.
Before getting their prescriptions, half of the men were told about a small risk of erectile dysfunction and other sexual side effects; none of the men had a history of such troubles. Of the men who got the warning, about 44 percent developed these symptoms. But among those kept in the dark, the rate was far lower: only about 15 percent.
It's known as the nocebo effect: When patients are warned of possible pain and unpleasant side effects, it increases the likelihood that they'll experience them.
If the placebo effect—in which positive patient expectations can be therapeutic—is relatively famous and well explored, the nocebo effect is its little-known evil twin.