Why do some people faint at the sight of a needle? Or start to sweat as soon as they walk into a dentist’s office? The answer could be the nocebo effect.

The nocebo effect is the mirror image of the better-documented placebo effect. In Latin, nocebo means “I will harm,” while placebo means “I will please.” A placebo can enhance healing or pain relief, while a nocebo has the opposite effect — making people feel worse.

The May 2011 issue of the Harvard Mental Health Letter examines several factors that contribute to the nocebo effect.

Conditioning. When people have had a negative experience or developed side effects in the past, they may do so again in response to sights, sounds, or other cues associated with that treatment. Such “conditioning” helps explain why as many as one in three people become nauseated or even vomit on entering a room where they have recently received chemotherapy.

Context. Medications and other treatments take on symbolic features — what psychologists call “context” — that can have nocebo effects. Red, orange, and yellow are colors associated with stimulation, while blue and green suggest sedation. In studies of sugar pills, people who take blue pills were more likely to say they feel drowsy afterward than people who took pink pills.

Suggestion. The use of certain words — such as warning people that a mild electric shock might hurt a great deal — can actually increase perception of pain severity.

It remains unclear what biological mechanisms are at work during the nocebo response. Dr. Michael Miller, editor in chief of the Harvard Mental Health Letter, notes that one theory is that — just as a placebo activates endorphins in the brain to provide pain relief — so too a nocebo may activate other receptors that stimulate the production of stress hormones like cortisol and in other ways affect perception of pain.

Read the full-length article: "New insights into the nocebo response"