People who believed dummy treatments were fighting their pain showed reduced brain activity in areas known to be involved in feeling pain, researchers at Princeton and the University of Michigan said on Thursday.
"What we have shown here in this study is when the placebo effect occurs, there really is something going on in the brain to reduce sensation," Dr Kenneth Casey a neurologist at the University of Michigan and the Ann Arbor Veteran Affairs Health Care System said.
The study is in line with other brain research that shows the placebo effect also causes changes in the brains of patients with, for example, depression.
The placebo effect has been known for centuries and the word itself comes from the Latin for "I shall please." Doctors for centuries have prescribed sugar pills to patients they could not otherwise help.
The placebo effect is so strong that medical studies usually must include a "placebo arm" to make sure a new drug truly is working through a unique mechanism.
An estimated 30% of patients with a range of conditions will get better simply from the action of taking a pill, getting a shot or otherwise receiving medical treatment.
But if the effects are psychological, they are certainly real. Casey's team has shown that placebos can affect the brain areas that cause the sensation of pain.
"Brain activity is a significant determinant of what we feel, how we feel and in this case of pain, how much we feel," Casey said.
"When people are expecting the pain to be less, the pain pathways – those areas in the brain that we know are activated by pain - show less activity, even though the stimulus is the same."