A prominent placebo researcher, Dr. Fabrizio Benedetti, was able to show just how peculiar the placebo effect really is. After inducing pain in participants for seven days whilst treating them with morphine, Benedetti secretly switched the pain medication to salt water. Luckily for him, the participants’ reports of pain went unchanged. Then things got weirder. Benedetti didn’t want to stop there, so he [secretively] gave the participants a morphine blocker and, bizarrely, the participants found that their pain returned, suggesting a form of biochemical reaction to the salt water placebo.
So you give people morphine and it works (to block pain). Switch it secretly with salt water and it still works. Secretly add a real morphine blocker and it no longer works. Bizarre.