EVERYONE knows that a placebo — a fake medication or sham procedure, typically used as a control in a medical trial — can nonetheless have a positive effect, relieving real symptoms like pain, bloating or a depressed mood. The placebo effect is a result of the patient’s expectation that the treatment will help.

But expectations can also do harm. When a patient anticipates a pill’s possible side effects, he can suffer them even if the pill is fake. This “nocebo” effect has been largely overlooked by researchers, clinicians and patients. In an article recently published in the journal Deutsche Ärzteblatt International, we and our colleague Ernil Hansen reviewed 31 studies, conducted by us and other researchers, that demonstrated the nocebo effect. We urge doctors and nurses to be more mindful of its dangers, particularly when informing patients about a treatment’s potential complications.

Consider the number of people in medical trials who, though receiving placebos, stop participating because of side effects. We found that 11 percent of people in fibromyalgia drug trials who were taking fake medication dropped out of the studies because of side effects like dizziness or nausea. Other researchers reported that the discontinuation rates because of side effects in placebo groups in migraine or tension drug trials were as much as 5 percent. Discontinuation rates in trials for statins ranged from 4 percent to 26 percent.

In a curious study, a team of Italian gastroenterologists asked people with and without diagnosed lactose intolerance to take lactose for an experiment on its effects on bowel symptoms. But in reality the participants received glucose, which does not harm the gut. Nonetheless, 44 percent of people with known lactose intolerance and 26 percent of those without lactose intolerance complained of gastrointestinal symptoms.