Why placebo pills work even when you know they’re a placebo

Darwin A Guevarra and Kari A Leibowitz in Psyche:

In a foundational study in 2010, researchers at Harvard Medical School randomised patients experiencing irritable bowel syndrome (IBS) symptoms into either an open-label placebo group or a no-treatment control group – and crucially, all the patients knew which group they were in. The researchers told patients in the open-label placebo group that the placebo effect is powerful, that the body can respond automatically to taking placebo pills (similar to the classic conditioning example of Pavlov’s dogs, who salivated at the sound of the dinner bell), that a positive attitude helps but is not required, and that it is vital to take the pills faithfully for the entire 21-day study period, regardless of their belief in the pills. By the end of the study, even though the placebo pills contained no active ingredients, and despite the patients knowing they’d been taking placebos, they reported fewer IBS symptoms and more improvement in overall quality of life than patients in the no-treatment control group.

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