Patients can benefit from being treated with sham drugs even if they are told they contain no active ingredient, scientists have found. The finding suggests that the placebo effect could work without the need for any deception on the part of the doctor, as had been previously thought.

When a patient undergoes a sham treatment for a disorder - such as taking a sugar pill - but still experiences a measurable improvement in their condition this is known as the placebo effect. It was widely thought, however, that the effect only works if the patient believes that the treatment they are receiving contains an active ingredient. Dummy treatments that might elicit the placebo effect are often used in clinical trials as a comparison group to allow scientists to measure the additional effects of experimental medicines.

To investigate the limits of placebo, Prof Ted Kaptchuk of Harvard Medical School's Osher Research Center divided 80 patients suffering from irritable bowel syndrome (IBS) into two groups: one received no treatment and the other was given dummy pills to take twice a day. The second group was told by the doctors that they would be taking "placebo pills made of an inert substance, like sugar pills, that have been shown in clinical studies to produce significant improvement in IBS-symptoms through mind-body self-healing processes".

"Not only did we make it absolutely clear that these pills had no active ingredient and were made from inert substances, but we actually had 'placebo' printed on the bottle," said Kaptchuk. "We told the patients that they didn't have to even believe in the placebo effect. Just take the pills."

The results, published in the scientific journal PLoS ONE, showed that the placebo pills were more effective at relieving symptoms compared with doing nothing at all.

"I didn't think it would work," says senior author Anthony Lembo of the Beth Israel Deaconess Medical Center and an expert on IBS. "I felt awkward asking patients to literally take a placebo. But to my surprise, it seemed to work for many of them."

In the trial, the IBS patients were monitored for three weeks. The group on placebo pills showed improvements both at the half way point and at the end of the period, when compared with the no-treatment group. By the end of the trial, almost twice as many patients treated with placebo showed adequate relief of symptoms compared with the control group. Patients taking the placebo also doubled their average rate of improvement to that achieved with the most powerful IBS medications.

"Directly harnessing placebo effects in a clinical setting has been problematic because of a widespread belief that beneficial responses to placebo treatment require concealment or deception," wrote the researchers in PLoS ONE. "This belief creates an ethical conundrum: to be beneficial in clinical practice placebos require deception but this violates the ethical principles of respect for patient autonomy and informed consent."

Nevertheless, a recent survey of doctors in the US, published in the British Medical Journal, found that while only small numbers of American physicians use inert placebo pills and injections, around half prescribed treatments that they considered to have no specific effect on patients' conditions and are used solely as placebos. "Given this situation, finding effective means of harnessing placebo responses in clinical practice without deception is a high priority," wrote the researchers.

Kaptchuk and his colleagues warned that there were limitations with their study, given its small size and limited time span. They called their result a "proof-of-concept" pilot study that would need to be repeated with larger numbers of people and over longer periods, to investigate the long-term effects of the placebo.

"Nevertheless," said Kaptchuk, "these findings suggest that rather than mere positive thinking, there may be significant benefit to the very performance of medical ritual. I'm excited about studying this further. Placebo may work even if patients know it is a placebo."