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A person’s reaction to a painkiller can be influenced by how a healthcare provider manages the individual’s expectations when therapy is administered, a new study finds. Pain experts say the findings could have a big impact on patient care and how drugs are tested.

British researchers observed 22 patients complaining of leg pain who were asked to rate their pain on a scale from one to 100. These patients were also hooked up to IVs to receive pain meds. At the start, the average pain rating was 66. Without knowing it, all the patients were given the strong painkiller remifentanil. Unaware that they’d received a treatment, the patients noted their pain level went down to 55. Then, when they were informed that they received a painkiller, their pain rating dropped to 39. Doctors then continued to give patients the same painkiller dose, but some study subjects were told the medication was being withdrawn. Those individuals then reported that the pain level jumped to 64.

“It’s another piece of evidence that we get what we expect in life,” said health researcher George Lewith. “It completely blows cold randomized clinical trials, which don’t take into account expectation.” The study was published in Science Translational Medicine.