Image caption Placebo acupuncture looks real, but the needles don't go into the body

Why some people respond to treatments that have no active ingredients in them may be down to their genes, a study in the journal PLoS ONE suggests.

The so-called "placebo effect" was examined in 104 patients with irritable bowel syndrome (IBS) in the US.

Those with a particular version of the COMT gene saw an improvement in their health after placebo acupuncture.

The scientists warn that while they hope their findings will be seen in other conditions, more work is needed.

Edzard Ernst, a professor of complementary medicine at the University of Exeter, said: "This is a fascinating but very preliminary result.

"It could solve the age-old question of why some individuals respond to placebo, while others do not.

"And if so, it could impact importantly on clinical practice.

"But we should be cautious - the study was small, we need independent replications, and we need to know whether the phenomenon applies just to IBS or to all diseases."

Gene variants

The placebo effect is when a patient experiences an improvement in their condition while undergoing an inert treatment such as taking a sugar pill or, in this case, placebo acupuncture, where the patient believes they are receiving acupuncture but a sham device prevents the needles going into their body.

While this is a very interesting work, what we have learned in the past few years is that there is not a single placebo response and a single mechanism Prof Fabrizio Benedetti , University of Turin Medical School

Two groups in the study had this type of treatment. One group received it in a business-like clinical manner and the other from a warm supportive practitioner. A third randomly chosen group received no treatment at all.

After three weeks the patients were asked if they had seen an improvement in their IBS, a common gastrointestinal disease that can cause abdominal pain and discomfort.

The team then used blood samples to look at what variant the individual had of the catechol-O-methyltranferase (COMT) gene. This plays a role in the dopamine pathway, a chemical known to produce a feel-good state.

Placebo dosage

Paper author Dr Kathryn Hall, from the Beth Israel Deaconess Medical Center (BIDMC), said this gene had been chosen because "there has been increasing evidence that the neurotransmitter dopamine is activated when people anticipate and respond to placebos".

The researchers found individuals with a COMT variant that triples the amount of dopamine in the front of the brain felt no improvement without treatment but an improvement with the placebo acupuncture.

Ted Kaptchuk, director of the Program in Placebo Studies and Therapeutic Encounter at BIDMC, said: "We wanted to tease apart the different doses of placebo.

"We got an effect in individuals with this specific genetic signature for the general placebo, but an even bigger effect in the elaborate placebo where warmer care was given.

"You can really see the advantage of a positive doctor-patient relationship."

Fabrizio Benedetti, professor of neurophysiology at the University of Turin Medical School, Italy, warned that dopamine may not be the only chemical involved with the placebo effect.

"A previous study on the genetics of placebo in social anxiety disorder showed that it is serotonin that is associated to placebo responsiveness and not dopamine," he said.

"While this is a very interesting work, what we have learned in the past few years is that there is not a single placebo response and a single mechanism, but many, across different medical conditions and therapeutic interventions."