Stuart Mazzone, of the University of Queensland in Australia, and colleagues found that, on average, people given a placebo had 45 percent less of an urge to cough when inhaling capsaicin than they did when they did not receive placebo.

"It's as if the brain has some occasion to control when to cough or not," said Omer Van den Bergh, a professor at the University of Leuven in Belgium, who was not involved in this study.

Ronald Eccles, the director of the Common Cold Centre at Cardiff University in the U.K., said that the study supports earlier work that placebos work just about as well to reduce coughing as anti-cough medicines.

"I think the key point is that if patients believe in a cough treatment then it does work for them," Eccles told Reuters Health by email.

ANESTHETIC VS. IRRITANT

An emerging idea among cough researchers is that coughing is not only a simple reflex, but is regulated by other areas of the brain that can be influenced by expectations that a substance can soothe the cough.

Mazzone's group sought to understand whether a placebo could also influence the urge to cough, which is the perception of irritation in the airways that precedes coughing.

The researchers asked 21 people to participate in a study in which they inhaled capsaicin, a component of chili peppers that makes people cough.

After finding how much capsaicin each person needed to inhale to get the urge to cough, the researchers made the participants believe they would be getting lidocaine -- a local anesthetic that make the chemical less irritating -- or a placebo as treatment.

In realty, however, all participants inhaled only inert gases that wouldn't impact their urge to cough.

After inhaling the inert gases, the participants inhaled full doses of capsaicin and asked whether they thought the "treatment" helped suppress their urge to cough.

In the first instance when the people were given the placebo, nine of them reported that they had less of an urge to cough than a group who were knowingly given a full-dose of capsaicin without any "medicine."

On average, the participants had a 45 percent reduction in the urge to cough -- from about a four on a 10-point scale to roughly a two.

"I was a little surprised by the magnitude of the response. It was very large," Mazzone told Reuters Health.

WHY SO STRONG?

Small studies have suggested that many other conditions respond to placebos, from depression to urinary problems to pain.

A recent study found that in some people, for example, depression responds well to placebo -- sometimes just as much as to talk therapy or antidepressant medications (see Reuters Health report of December 21, 2011).

Mazzone said the placebo response in cough appears to be even greater than in other health conditions.

A placebo's effect on pain, for instance, typically only reduces pain sensations by about 25 to 30 percent, compared to the 45 percent he saw in his study, Mazzone told Reuters Health.

"It's difficult to know why that is" that cough responds so well to placebo," he added, speculating that the cough reflex is less hard-wired than the pain reflex, because the latter is "so critical for survival."

SOURCE: bit.ly/KtaNyp CHEST, online May 31, 2012.

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