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When a patient takes a sugar pill and feels better, it is tempting to dismiss the improvement as imaginary. Yet the placebo effect, the improvement people experience after a treatment with no active ingredient, is one of the most thoroughly studied phenomena in medicine, and it is considerably more interesting than the word "imaginary" suggests.
The modern study of placebos is often traced to an influential article published in 1955 by the American doctor Henry Beecher, who claimed that around a third of patients improved when given a placebo. His figures have since been criticised. Some of the "placebo effect" in his examples was probably due to other factors: many conditions improve naturally over time, and patients who are unusually unwell when they enter a study tend to be closer to normal when measured again. Nevertheless, careful research has confirmed that placebos can have genuine effects, particularly on symptoms such as pain and nausea.
These effects are not simply a matter of patients telling doctors what they want to hear. Brain-imaging studies have shown that, in some people, taking a placebo painkiller is accompanied by changes in brain activity in areas involved in processing pain. Expectations, it seems, can influence how the brain interprets signals from the body. The context of treatment also matters: in some studies, injections have produced larger placebo effects than pills, and more expensive-looking pills have outperformed cheaper-looking ones.
Most surprisingly, placebos may work even when patients know they are taking them. In a 2010 study led by Ted Kaptchuk at Harvard, patients with irritable bowel syndrome were openly given pills described as containing no medicine. Those who took them reported greater improvement in their symptoms than a group who received no treatment. Researchers have suggested that the ritual of taking a pill, together with a supportive relationship with a doctor, may itself be therapeutic.
The flip side is the nocebo effect: when people expect a treatment to cause harm, they may experience side effects even from an inactive pill. In clinical trials, a substantial share of patients receiving placebos report side effects such as headaches or tiredness, often those listed on the consent form they were given.
It is important not to exaggerate. Placebos do not shrink tumours, cure infections or repair broken bones. But the evidence suggests that the way treatments are delivered, including how doctors speak to patients and what patients are led to expect, can make a real difference to how people feel. Rather than a nuisance to be eliminated from experiments, the placebo effect may be something medicine can learn to use ethically.