The placebo effect refers to the phenomenon that patients unknowingly take fake drugs that have no effect at all, but can get the same or even better effect as the real drug  The placebo effect is supported by many clinical studies
One explanation for this phenomenon is that people's expectations for the future will change the physiological state of the brain, which in turn causes physiological changes in the whole body
The subject was injected with saline and convinced that it was a painkiller, and the subject's brain then secreted the painkiller endorphin