The susceptibility of patients to the placebo effect has been linked to a gene by researchers in Sweden. The findings, published in the Journal of Neuroscience, provide clues as to how some patients can respond to taking a placebo, usually a sugar pill, instead of the real drug.
The group of researchers at Uppsala University and Gothenburg University examined 108 individuals undergoing an eight-week treatment for social anxiety disorder, an exaggerated fear of public humiliation. They had to give a stressful presentation at the beginning and end of the treatment while their brain activity was measured using a PET (positron emission tomography) brain camera. It was a placebo double-blind study, in which 25 of the patients were given a sugar pill instead of the drug, but neither the patients nor the researchers knew who had been given the placebo or the drug.
The results they collected showed that 40 per cent of the placebo group showed the same degree of anxiety relief from the sugar pill as the group who actually took the drug. The brain scans of these patients who responded to the placebo showed that activity in the amygdala, the brain's 'fear centre', had dropped by three per cent.
Tomas Furmark, of the Uppsala University Department of Psychology, directed the study. He then investigated the genes for tryptophan hydroxylase-2, which makes the brain chemical serotonin, and the serotonin transporter. These genes had been linked to fear by an earlier study, which showed that people with two copies of a specific variant of the tryptophan hydroxlyase-2 gene were less anxious. Furmark found that only the participants who had two copies of the variant were responsive to the placebo pill, and that the presence of these variants could predict the degree of anxiety relief and moderation of the amygdala.
Furmark believes that the effect of the genes may be involved in the placebo effect in other conditions in which the amagdala is involved such as phobias and depression, but that further clinical studies will need to be done to find out. He says: 'the findings show that the possibility of demonstrating that an active treatment functions better than a placebo can be affected by gene variants in the trail subjects. It is also possible that genes can explain why certain people respond well or poorly to anxiety-moderating drugs and psychotherapy respectively'.
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