Zubieta said that patients may have had an expectation of improvement after hearing that placebos showed 'self-healing processes.'
"You're telling them they're receiving an inactive substance, but you're also telling them that they might get better," said Zubieta. "I'd like to know how much of that expectation affected the improved outcomes."
In the study, authors noted some limitations, which included a relatively small sample size and a short trial duration which would make it difficult to estimate long-term results.
Plus, the improvements were self-reported. Placebo effects on pain and depression are often measured through PET and MRI scans, imaging techniques that measure chemical and metabolic changes in the brain. But when the improvements are measured by the patient, Zubieta said it lacks the objective measure to be sure of the results.
Dr. John Clarke, assistant professor of medicine in the division of gastroenterology at Johns Hopkins School of Medicine, said that the proactive and regimented nature of taking a placebo may in itself be related to improved symptoms.
"Some remedies of IBS are just stress reduction, so if giving people a placebo decreases their stress level and makes them feel proactive, then you could argue that it's working," said Clarke.